rebase: fix handling of restrict_revision

restrict_revision in the original shell script was an excluded revision
range.  It is also treated that way by the am-backend.  In the
conversion from shell to C (see commit 6ab54d1 ("rebase -i:
implement the logic to initialize $revisions in C", 2018-08-28)), the
interactive-backend accidentally treated it as a positive revision
rather than a negated one.

This was missed as there were no tests in the testsuite that tested an
interactive rebase with fork-point behavior.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -348,8 +348,8 @@ static int do_interactive_rebase(struct rebase_options *opts, unsigned flags)
 
 	argv_array_pushl(&make_script_args, "", revisions, NULL);
 	if (opts->restrict_revision)
-		argv_array_push(&make_script_args,
-				oid_to_hex(&opts->restrict_revision->object.oid));
+		argv_array_pushf(&make_script_args, "^%s",
+				 oid_to_hex(&opts->restrict_revision->object.oid));
 
 	ret = sequencer_make_script(the_repository, &todo_list.buf,
 				    make_script_args.argc, make_script_args.argv,

t/t3400-rebase.sh

@@ -165,11 +165,29 @@ test_expect_success 'rebase works with format.useAutoBase' '
 	git rebase master
 '
 
-test_expect_success 'default to common base in @{upstream}s reflog if no upstream arg' '
+test_expect_success 'default to common base in @{upstream}s reflog if no upstream arg (--merge)' '
 	git checkout -b default-base master &&
 	git checkout -b default topic &&
 	git config branch.default.remote . &&
 	git config branch.default.merge refs/heads/default-base &&
+	git rebase --merge &&
+	git rev-parse --verify default-base >expect &&
+	git rev-parse default~1 >actual &&
+	test_cmp expect actual &&
+	git checkout default-base &&
+	git reset --hard HEAD^ &&
+	git checkout default &&
+	git rebase --merge &&
+	git rev-parse --verify default-base >expect &&
+	git rev-parse default~1 >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'default to common base in @{upstream}s reflog if no upstream arg' '
+	git checkout -B default-base master &&
+	git checkout -B default topic &&
+	git config branch.default.remote . &&
+	git config branch.default.merge refs/heads/default-base &&
 	git rebase &&
 	git rev-parse --verify default-base >expect &&
 	git rev-parse default~1 >actual &&