rebase: default to using the builtin rebase

Now that the builtin rebase is feature-complete, we should use it by
default. Let's keep the legacy scripted version around for the time
being; Once the builtin rebase is well-tested enough, we can remove
`git-legacy-rebase.sh`.

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -54,7 +54,7 @@ static int use_builtin_rebase(void)
 	cp.git_cmd = 1;
 	if (capture_command(&cp, &out, 6)) {
 		strbuf_release(&out);
-		return 0;
+		return 1;
 	}
 
 	strbuf_trim(&out);