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

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

When git-merge updates HEAD as a result of a merge record what
happened during the merge into the reflog associated with HEAD
(if any).  The log reports who caused the update (git-merge or
git-pull, by invoking git-merge), what the remote ref names were
and the type of merge process used.

The merge information can be useful when reviewing a reflog for
a branch such as `master` where fast forward and trivial in index
merges might be common as the user tracks an upstream.

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