Given a list of <pack>.idx files, this command validates the
index file and the corresponding .pack file for consistency.

This patch also uses the same validation mechanism in fsck-cache
when the --full flag is used.

During normal operation, sha1_file.c verifies that a given .idx
file matches the .pack file by comparing the SHA1 checksum
stored in .idx file and .pack file as a minimum sanity check.
We may further want to check the pack signature and version when
we map the pack, but that would be a separate patch.

Earlier, errors to map a pack file was not flagged fatal but led
to a random fatal error later.  This version explicitly die()s
when such an error is detected.
Signed-off-by: NJunio C Hamano <junkio@cox.net>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>