Fix double "close()" in ce_compare_data

Doing an "strace" on "git diff" shows that we close() a file descriptor
twice (getting EBADFD on the second one) when we end up in ce_compare_data
if the index does not match the checked-out stat information.

The "index_fd()" function will already have closed the fd for us, so we
should not close it again.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>