builtin/rebase.c: fix "options.onto_name" leak

Similar to the existing "squash_onto_name" added in [1] we need to
free() the xstrdup()'d "options.onto.name" added for "--keep-base" in
[2]..

1. 9dba809a69 (builtin rebase: support --root, 2018-09-04)
2. 414d924beb (rebase: teach rebase --keep-base, 2019-08-27)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -1039,6 +1039,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	struct string_list strategy_options = STRING_LIST_INIT_NODUP;
 	struct object_id squash_onto;
 	char *squash_onto_name = NULL;
+	char *keep_base_onto_name = NULL;
 	int reschedule_failed_exec = -1;
 	int allow_preemptive_ff = 1;
 	int preserve_merges_selected = 0;
@@ -1675,7 +1676,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		strbuf_addstr(&buf, options.upstream_name);
 		strbuf_addstr(&buf, "...");
 		strbuf_addstr(&buf, branch_name);
-		options.onto_name = xstrdup(buf.buf);
+		options.onto_name = keep_base_onto_name = xstrdup(buf.buf);
 	} else if (!options.onto_name)
 		options.onto_name = options.upstream_name;
 	if (strstr(options.onto_name, "...")) {
@@ -1851,6 +1852,7 @@ cleanup:
 	free(options.strategy);
 	strbuf_release(&options.git_format_patch_opt);
 	free(squash_onto_name);
+	free(keep_base_onto_name);
 	string_list_clear(&strategy_options, 0);
 	return !!ret;
 }

t/t3416-rebase-onto-threedots.sh

@@ -5,6 +5,7 @@ test_description='git rebase --onto A...B'
 GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 . "$TEST_DIRECTORY/lib-rebase.sh"