fsck: exit with non-zero status upon errors

fsck: exit with non-zero status upon errors

git-fsck always exited with status 0, which was a bit sloppy.
This makes it exit with a non-zero status when errors are
found.  The error code is an OR'ed result of:

  1 if corrupted objects are found.
  2 if objects that are ought to be reachable are missing or corrupt.

For example, it would exit with 1 in a repository with an
unreachable corrupt object.  If a tree object of the HEAD commit
is corrupt, you would get 3.

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