Record the type of commit operation in the reflog.

Record the type of commit operation in the reflog.

If committing a merge (.git/MERGE_HEAD exists), an initial tree
(no HEAD) or using --amend to amend the prior commit then denote
the subtype of commit in the reflog.  This helps to distinguish
amended or merge commits from normal commits.

In the case of --amend the prior sha1 is probably the commit which
is being thrown away in favor of the new commit.  Since it is likely
that the old commit doesn't have any ref pointing to it anymore
it can be interesting to know why that the commit was replaced
and orphaned.

In the case of a merge the prior sha1 is probably the first parent
of the new merge commit.  Consequently having its prior sha1 in the
reflog is slightly less interesting but its still informative to
know the commit was the result of a merge which had to be completed
by hand.

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