completion: tab completion of filenames for 'git restore'

completion: tab completion of filenames for 'git restore'

If no --args are present after 'git restore', it assumes that you
want to tab-complete one of the files with unstaged uncommitted
changes.

If a file has been staged, we don't want to list it, as restoring those
requires a slightly more complex `git restore --staged`, so we only list
those files that are --modified. While --committable also looks like
a good candidate, that includes changes that have been staged.

Signed-off-by: David Cantrell <david@cantrell.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>