…l with empty files

The POSIX unified diff format does not support them (empty file = absent file)
but the git extensions "deleted file mode" and "new file mode" do.
This also adds support for deleting files without have to knew their
content.

Adding support for these extensions requires specialised variants
to be able to print both the "[deleted|new] file mode <filename>" line
and the "git --diff ..." header line.

…before-archiving-phase1

…ace modes