Teach "apply --index-info" to handle rename patches

Teach "apply --index-info" to handle rename patches

With v1.5.3.2~14 (apply --index-info: fall back to current index for
mode changes, 2007-09-17), git apply learned to stop worrying
about the lack of diff index line when a file already present in the
current index had no content change.

But it still worries too much: for rename patches, it is checking
that both the old and new filename are present in the current
index.  This makes no sense, since a file rename generally
involves creating a file there was none before.

So just check the old filename.

Noticed while trying to use “git rebase” with diff.renames = copies.

[jn: add tests]

Reported-by: David D. Kilzer <ddkilzer@kilzer.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>