apply --index-info: fall back to current index for mode changes

apply --index-info: fall back to current index for mode changes

"git diff" does not record index lines for pure mode changes (i.e. no
lines changed).  Therefore, apply --index-info would call out a bogus
error.

Instead, fall back to reading the info from the current index.

Incidentally, this fixes an error where git-rebase would not rebase a
commit including a pure mode change, and changes requiring a threeway
merge.

Noticed and later tested by Chris Shoemaker.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>