docs & comments: replace mentions of "git-add--interactive.perl"

docs & comments: replace mentions of "git-add--interactive.perl"

Now that we've removed "git-add--interactive.perl" let's replace
mentions of it with "add-interactive.c". In the case of the "git add"
documentation we were using it as an example filename, so the mention
wasn't wrong, but using a dead file is slightly confusing.

The "borrowed" comment here likewise isn't wrong, but let's mention
the successor file instead. In the case of pathspec.c the implied TODO
item should refer to the current code (and the comment may not even be
current, I didn't check).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>