git: fix leaking argv when handling builtins

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>