[PATCH] Verify index file before we opportunistically update it

This is a fix for the following bug:
http://thread.gmane.org/gmane.comp.version-control.git/245946/focus=245965

I added 2 functions: verify_index_from and verify_index. They return 1
if the sha1 is correct, otherwise 0. I choose to not die if any errors
are occurred because we just want to not proceed to "opportunistic
update".

Some questions:
1) Is it better to have these functions as static?
2) If the answer of (1) is no, should I define verify_cache*() also?
3) If something goes wrong in verify_hdr(), it will print an error
   message, should I make a "quietly" version of it?

Yiannis Marangos (1):
  Verify index file before we opportunistically update it

 cache.h      |  3 +++
 read-cache.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++---------
 2 files changed, 71 insertions(+), 11 deletions(-)

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1.9.1