Log ref changes made by git-fetch and git-pull.

When git-fetch updates a reference record in the associated reflog
what type of update took place and who caused it (git-fetch or
git-pull, by invoking git-fetch).

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