Log ref updates made by fetch.

Log ref updates made by fetch.

If a ref is changed by http-fetch, local-fetch or ssh-fetch
record the change and the remote URL/name in the log for the ref.
This requires loading the config file to check logAllRefUpdates.

Also fixed a bug in the ref lock generation; the log file name was
not being produced right due to a bad prefix length.

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