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>