rebase --rebase-merges: root commits can be cousins, too

Reported by Wink Saville: when rebasing with no-rebase-cousins, we
will want to refrain from rebasing all of them, even when they are
root commits.

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

sequencer.c

@@ -3903,7 +3903,8 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 		}
 
 		if (!commit)
-			fprintf(out, "%s onto\n", cmd_reset);
+			fprintf(out, "%s %s\n", cmd_reset,
+				rebase_cousins ? "onto" : "[new root]");
 		else {
 			const char *to = NULL;
 

t/t3430-rebase-merges.sh

@@ -287,5 +287,30 @@ test_expect_success 'a "merge" into a root commit is a fast-forward' '
 	test_cmp_rev HEAD $head
 '
 
+test_expect_success 'A root commit can be a cousin, treat it that way' '
+	git checkout --orphan khnum &&
+	test_commit yama &&
+	git checkout -b asherah master &&
+	test_commit shamkat &&
+	git merge --allow-unrelated-histories khnum &&
+	test_tick &&
+	git rebase -f -r HEAD^ &&
+	! test_cmp_rev HEAD^2 khnum &&
+	test_cmp_graph HEAD^.. <<-\EOF &&
+	*   Merge branch '\''khnum'\'' into asherah
+	|\
+	| * yama
+	o shamkat
+	EOF
+	test_tick &&
+	git rebase --rebase-merges=rebase-cousins HEAD^ &&
+	test_cmp_graph HEAD^.. <<-\EOF
+	*   Merge branch '\''khnum'\'' into asherah
+	|\
+	| * yama
+	|/
+	o shamkat
+	EOF
+'
 
 test_done