rebase --rebase-merges: a "merge" into a new root is a fast-forward

When a user provides a todo list containing something like

	reset [new root]
	merge my-branch

let's do the same as if pulling into an orphan branch: simply
fast-forward.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -2870,6 +2870,18 @@ static int do_merge(struct commit *commit, const char *arg, int arg_len,
 		goto leave_merge;
 	}
 
+	if (opts->have_squash_onto &&
+	    !oidcmp(&head_commit->object.oid, &opts->squash_onto)) {
+		/*
+		 * When the user tells us to "merge" something into a
+		 * "[new root]", let's simply fast-forward to the merge head.
+		 */
+		rollback_lock_file(&lock);
+		ret = fast_forward_to(&merge_commit->object.oid,
+				       &head_commit->object.oid, 0, opts);
+		goto leave_merge;
+	}
+
 	if (commit) {
 		const char *message = get_commit_buffer(commit, NULL);
 		const char *body;

t/t3430-rebase-merges.sh

@@ -275,4 +275,17 @@ test_expect_success 'root commits' '
 	test_cmp_rev HEAD $before
 '
 
+test_expect_success 'a "merge" into a root commit is a fast-forward' '
+	head=$(git rev-parse HEAD) &&
+	cat >script-from-scratch <<-EOF &&
+	reset [new root]
+	merge $head
+	EOF
+	test_config sequence.editor \""$PWD"/replace-editor.sh\" &&
+	test_tick &&
+	git rebase -i -r HEAD^ &&
+	test_cmp_rev HEAD $head
+'
+
+
 test_done