Before writing the shallow file, we stat() the existing file
to make sure it has not been updated since our operation
began. However, we do not do so under a lock, so there is a
possible race:

  1. Process A takes the lock.

  2. Process B calls check_shallow_file_for_update and finds
     no update.

  3. Process A commits the lockfile.

  4. Process B takes the lock, then overwrite's process A's
     changes.

We can fix this by doing our check while we hold the lock.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>