Fix automerge message.

The rewrite done while adding the shorthand support made the remote
refname recorded in the commit message too long for human consumption,
while losing information by using the shorthand not the real URL to
name the remote repository there.  They were both bad changes done
without enough thinking.

Pointed out by Linus.

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