git: fix leaking argv when handling builtins

In `handle_builtin()` we may end up creating an ad-hoc argv array in
case we see that the command line contains the "--help" parameter. In
this case we observe two memory leaks though:

  - We leak the `struct strvec` itself because we directly exit after
    calling `run_builtin()`, without bothering about any cleanups.

  - Even if we free'd that vector we'd end up leaking some of its
    strings because `run_builtin()` will modify the array.

Plug both of these leaks.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -711,6 +711,7 @@ static void strip_extension(const char **argv)
 static void handle_builtin(int argc, const char **argv)
 {
 	struct strvec args = STRVEC_INIT;
+	const char **argv_copy = NULL;
 	const char *cmd;
 	struct cmd_struct *builtin;
 
@@ -731,13 +732,28 @@ static void handle_builtin(int argc, const char **argv)
 		}
 
 		argc++;
-		argv = args.v;
+
+		/*
+		 * `run_builtin()` will modify the argv array, so we need to
+		 * create a shallow copy such that we can free all of its
+		 * strings.
+		 */
+		CALLOC_ARRAY(argv_copy, argc + 1);
+		COPY_ARRAY(argv_copy, args.v, argc);
+
+		argv = argv_copy;
 	}
 
 	builtin = get_builtin(cmd);
-	if (builtin)
-		exit(run_builtin(builtin, argc, argv));
+	if (builtin) {
+		int ret = run_builtin(builtin, argc, argv);
+		strvec_clear(&args);
+		free(argv_copy);
+		exit(ret);
+	}
+
 	strvec_clear(&args);
+	free(argv_copy);
 }
 
 static void execv_dashed_external(const char **argv)