[Linux] Implement update methods on Linux for placeholder files/dirs

For the Linux implementation of the equivalent of the PrjFSLib.Mac
DeleteFile(), UpdatePlaceholderIfNeeded(), and
ReplacePlaceholderFileWithSymLink() methods, we rely on the
projfs_get_attrs() function in our libprojfs library, which allows us to
read the "user.projection.empty" xattr. If this xattr exists, the file or
directory is not fully local, and may be deleted.

Our implementation of these functions follows the logic in the Mac
C++ code, including the small races in the DeleteFile() method
between performing a stat(2) to check the nature of the file/dir, then
a getxattr(2) to look for our projection flag xattr, and finally a File.Delete()
or Directory.Delete().

We detect the case where an inode is neither a file nor a directory
by introducing an Unknown projection state and reporting this as
a "full file" condition to our caller, again parallel to the Mac C++ code.

We handle requests to delete the repository's root directory by returning
EDirectoryNotEmpty, instead of ignoring them and returning Success.
This is necessary to avoid short-circuiting the logic of the calling
UpdatePlaceholders() method in GitIndexProjection.

Separately, we ensure Linux platform CI builds succeed by ignoring
the Profiling(Release) builds (as done for Mac as well) and searching
the usual system locations for our libprojfs headers and library.

[Linux] Implement update methods on Linux for placeholder files/dirs

For the Linux implementation of the equivalent of the PrjFSLib.Mac
DeleteFile(), UpdatePlaceholderIfNeeded(), and
ReplacePlaceholderFileWithSymLink() methods, we rely on the
projfs_get_attrs() function in our libprojfs library, which allows us to
read the "user.projection.empty" xattr. If this xattr exists, the file or
directory is not fully local, and may be deleted.

Our implementation of these functions follows the logic in the Mac
C++ code, including the small races in the DeleteFile() method
between performing a stat(2) to check the nature of the file/dir, then
a getxattr(2) to look for our projection flag xattr, and finally a File.Delete()
or Directory.Delete().

We detect the case where an inode is neither a file nor a directory
by introducing an Unknown projection state and reporting this as
a "full file" condition to our caller, again parallel to the Mac C++ code.

We handle requests to delete the repository's root directory by returning
EDirectoryNotEmpty, instead of ignoring them and returning Success.
This is necessary to avoid short-circuiting the logic of the calling
UpdatePlaceholders() method in GitIndexProjection.

Separately, we ensure Linux platform CI builds succeed by ignoring
the Profiling(Release) builds (as done for Mac as well) and searching
the usual system locations for our libprojfs headers and library.

[Linux] Implement update methods on Linux for placeholder files/dirs

For the Linux implementation of the equivalent of the PrjFSLib.Mac
DeleteFile(), UpdatePlaceholderIfNeeded(), and
ReplacePlaceholderFileWithSymLink() methods, we rely on the
projfs_get_attrs() function in our libprojfs library, which allows us to
read the "user.projection.empty" xattr. If this xattr exists, the file or
directory is not fully local, and may be deleted.

Our implementation of these functions follows the logic in the Mac
C++ code, including the small races in the DeleteFile() method
between performing a stat(2) to check the nature of the file/dir, then
a getxattr(2) to look for our projection flag xattr, and finally a File.Delete()
or Directory.Delete().

We detect the case where an inode is neither a file nor a directory
by introducing an Unknown projection state and reporting this as
a "full file" condition to our caller, again parallel to the Mac C++ code.

We handle requests to delete the repository's root directory by returning
EDirectoryNotEmpty, instead of ignoring them and returning Success.
This is necessary to avoid short-circuiting the logic of the calling
UpdatePlaceholders() method in GitIndexProjection.

Separately, we ensure Linux platform CI builds succeed by ignoring
the Profiling(Release) builds (as done for Mac as well) and searching
the usual system locations for our libprojfs headers and library.

[Linux] Implement update methods on Linux for placeholder files/dirs

For the Linux implementation of the equivalent of the PrjFSLib.Mac
DeleteFile(), UpdatePlaceholderIfNeeded(), and
ReplacePlaceholderFileWithSymLink() methods, we rely on the
projfs_get_attrs() function in our libprojfs library, which allows us to
read the "user.projection.empty" xattr. If this xattr exists, the file or
directory is not fully local, and may be deleted.

Our implementation of these functions follows the logic in the Mac
C++ code, including the small races in the DeleteFile() method
between performing a stat(2) to check the nature of the file/dir, then
a getxattr(2) to look for our projection flag xattr, and finally a File.Delete()
or Directory.Delete().

We detect the case where an inode is neither a file nor a directory
by introducing an Unknown projection state and reporting this as
a "full file" condition to our caller, again parallel to the Mac C++ code.

We handle requests to delete the repository's root directory by returning
EDirectoryNotEmpty, instead of ignoring them and returning Success.
This is necessary to avoid short-circuiting the logic of the calling
UpdatePlaceholders() method in GitIndexProjection.

Separately, we ensure Linux platform CI builds succeed by ignoring
the Profiling(Release) builds (as done for Mac as well) and searching
the usual system locations for our libprojfs headers and library.

[Linux] Implement update methods on Linux for placeholder files/dirs

For the Linux implementation of the equivalent of the PrjFSLib.Mac
DeleteFile(), UpdatePlaceholderIfNeeded(), and
ReplacePlaceholderFileWithSymLink() methods, we rely on the
projfs_get_attrs() function in our libprojfs library, which allows us to
read the "user.projection.empty" xattr. If this xattr exists, the file or
directory is not fully local, and may be deleted.

Our implementation of these functions follows the logic in the Mac
C++ code, including the small races in the DeleteFile() method
between performing a stat(2) to check the nature of the file/dir, then
a getxattr(2) to look for our projection flag xattr, and finally a File.Delete()
or Directory.Delete().

We detect the case where an inode is neither a file nor a directory
by introducing an Unknown projection state and reporting this as
a "full file" condition to our caller, again parallel to the Mac C++ code.

We handle requests to delete the repository's root directory by returning
EDirectoryNotEmpty, instead of ignoring them and returning Success.
This is necessary to avoid short-circuiting the logic of the calling
UpdatePlaceholders() method in GitIndexProjection.

Separately, we ensure Linux platform CI builds succeed by ignoring
the Profiling(Release) builds (as done for Mac as well) and searching
the usual system locations for our libprojfs headers and library.

[Linux] Implement update methods on Linux for placeholder files/dirs

For the Linux implementation of the equivalent of the PrjFSLib.Mac
DeleteFile(), UpdatePlaceholderIfNeeded(), and
ReplacePlaceholderFileWithSymLink() methods, we rely on the
projfs_get_attrs() function in our libprojfs library, which allows us to
read the "user.projection.empty" xattr. If this xattr exists, the file or
directory is not fully local, and may be deleted.

Our implementation of these functions follows the logic in the Mac
C++ code, including the small races in the DeleteFile() method
between performing a stat(2) to check the nature of the file/dir, then
a getxattr(2) to look for our projection flag xattr, and finally a File.Delete()
or Directory.Delete().

We detect the case where an inode is neither a file nor a directory
by introducing an Unknown projection state and reporting this as
a "full file" condition to our caller, again parallel to the Mac C++ code.

We handle requests to delete the repository's root directory by returning
EDirectoryNotEmpty, instead of ignoring them and returning Success.
This is necessary to avoid short-circuiting the logic of the calling
UpdatePlaceholders() method in GitIndexProjection.

Separately, we ensure Linux platform CI builds succeed by ignoring
the Profiling(Release) builds (as done for Mac as well) and searching
the usual system locations for our libprojfs headers and library.

[Linux] Implement update methods on Linux for placeholder files/dirs

For the Linux implementation of the equivalent of the PrjFSLib.Mac
DeleteFile(), UpdatePlaceholderIfNeeded(), and
ReplacePlaceholderFileWithSymLink() methods, we rely on the
projfs_get_attrs() function in our libprojfs library, which allows us to
read the "user.projection.empty" xattr. If this xattr exists, the file or
directory is not fully local, and may be deleted.

Our implementation of these functions follows the logic in the Mac
C++ code, including the small races in the DeleteFile() method
between performing a stat(2) to check the nature of the file/dir, then
a getxattr(2) to look for our projection flag xattr, and finally a File.Delete()
or Directory.Delete().

We detect the case where an inode is neither a file nor a directory
by introducing an Unknown projection state and reporting this as
a "full file" condition to our caller, again parallel to the Mac C++ code.

We handle requests to delete the repository's root directory by returning
EDirectoryNotEmpty, instead of ignoring them and returning Success.
This is necessary to avoid short-circuiting the logic of the calling
UpdatePlaceholders() method in GitIndexProjection.

Separately, we ensure Linux platform CI builds succeed by ignoring
the Profiling(Release) builds (as done for Mac as well) and searching
the usual system locations for our libprojfs headers and library.