Catch write_ref_sha1 failure in receive-pack

Catch write_ref_sha1 failure in receive-pack

This failure to catch the failure of write_ref_sha1 was noticed
by Bill Lear.  The ref will not update if the log file could not
be appended to (due to file permissions problems).  Such a failure
should be flagged as a failure to update the ref, so that the client
knows the push did not succeed.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>