git-reset to remove "$GIT_DIR/MERGE_MSG"

git-reset to remove "$GIT_DIR/MERGE_MSG"

An earlier commit a9cb3c6e changed git-commit to use the
contents of MERGE_MSG even when we do not have MERGE_HEAD (the
rationale is in its log message).

However, the change tricks the following sequence to include a
merge message in a completely unrelated commit:

	$ git pull somewhere
	: oops, the conflicts are too much.  forget it.
        $ git reset --hard
        : work work work
        $ git commit

To fix this confusion, this patch makes "git reset" to remove
the leftover MERGE_MSG that was prepared when the user abandoned
the merge.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Acked-by: Luben Tuikov <ltuikov@yahoo.com