Git-prune-script loses blobs referenced from an uncommitted cache.

Git-prune-script loses blobs referenced from an uncommitted cache.

(updated from the version posted to GIT mailing list).

When a new blob is registered with update-cache, and before the cache
is written as a tree and committed, git-fsck-cache will find the blob
unreachable.  This patch adds a new flag, "--cache" to git-fsck-cache,
with which it keeps such blobs from considered "unreachable".

The git-prune-script is updated to use this new flag.  At the same time
it adds .git/refs/*/* to the set of default locations to look for heads,
which should be consistent with expectations from Cogito users.

Without this fix, "diff-cache -p --cached" after git-prune-script has
pruned the blob object will fail mysteriously and git-write-tree would
also fail.

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