git-apply: parse index information

Add an new option --show-index-info to git-apply command to
summarize the index information new git-diff outputs.  The
command shows something similar to git-ls-files --stage output
for the pre-change image:

100644 7be5041...	apply.c
100644 ec2a161...	cache.h
...

Signed-off-by: Junio C Hamano <junkio@cox.net>