At present the summary of file types and sizes output by the
"migrate info" command is sorted only by the byte count totals.
When two entries have the same counts, though, the order in
which they are output is not defined, and may vary.  This will
pose a problem for some tests we expect to add in subsequent
commits.

Therefore we handle the case where two entries have identical
byte counts by further comparing the file types and returning
whichever sorts first lexigraphically, according to Go's
internal string sort order.

At present, the "git lfs migrate info" command reports
the total sizes, numbers, and file extensions for blobs
which have already been converted to LFS pointers, and
it uses the size of the pointer blobs themselves instead
of the size of object data referenced by the pointers.

The result is a confusing output which does not distinguish
these types, has misleading size totals, and generates
output listings even for repositories fully migrated to LFS.

We therefore add a new --pointers option to the "migrate info"
command so that a summary of all valid LFS pointers found in
the repository traversal may be either skipped entirely or
reported separately.

To support this option we add an lfs.DecodePointerFromBlob()
function which parallels the existing lfs.DecodePointerFromFile()
function.

We also refactor some of the entry lookup logic in the
BlobFn callback function used in the migrateInfoCommand()
function into a new findEntryByExtension() utility function.

We add a number of tests of the functionality of the new
--pointers option to the "migrate info" command in order
to confirm it works as expected in all modes, and both with
and without Git LFS pointers in the test repositories.

We also add tests of repositories where some file extensions
are tracked in .gitattributes files and the corresponding blobs
should be Git LFS pointers, but are not yet.  In these cases,
we do want to report these file extensions in summary data of
the output listing.

In order to ensure our tests succeed on Windows, we alter
several fixture setup functions to create the .gitattributes
files using "echo" instead of "git lfs track".  This keeps
the line endings consistent across platforms, and therefore
the .gitattributes file sizes in the "migrate info" summaries.

We update the manual page for the "git lfs migrate" command
to describe the use and purpose of the new --pointers option
of the "info" command mode.