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: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>