built-ins & libs & helpers: add/move destructors, fix leaks

built-ins & libs & helpers: add/move destructors, fix leaks

Fix various leaks in built-ins, libraries and a test helper here we
were missing a call to strbuf_release(), string_list_clear() etc, or
were calling them after a potential "return".

Comments on individual changes:

- builtin/checkout.c: Fix a memory leak that was introduced in [1]. A
  sibling leak introduced in [2] was recently fixed in [3]. As with [3]
  we should be using the wt_status_state_free_buffers() API introduced
  in [4].

- builtin/repack.c: Fix a leak that's been here since this use of
  "strbuf_release()" was added in a1bbc6c0176 (repack: rewrite the shell
  script in C, 2013-09-15). We don't use the variable for anything
  except this loop, so we can instead free it right afterwards.

- builtin/rev-parse: Fix a leak that's been here since this code was
  added in 21d47835386 (Add a parseopt mode to git-rev-parse to bring
  parse-options to shell scripts., 2007-11-04).

- builtin/stash.c: Fix a couple of leaks that have been here since
  this code was added in d4788af875c (stash: convert create to builtin,
  2019-02-25), we strbuf_release()'d only some of the "struct strbuf" we
  allocated earlier in the function, let's release all of them.

- ref-filter.c: Fix a leak in 482c1191869 (gpg-interface: improve
  interface for parsing tags, 2021-02-11), we don't use the "payload"
  variable that we ask parse_signature() to populate for us, so let's
  free it.

- t/helper/test-fake-ssh.c: Fix a leak that's been here since this
  code was added in 3064d5a38c7 (mingw: fix t5601-clone.sh,
  2016-01-27). Let's free the "struct strbuf" as soon as we don't need
  it anymore.

1. c45f0f525de (switch: reject if some operation is in progress,
   2019-03-29)
2. 2708ce62d21 (branch: sort detached HEAD based on a flag,
   2021-01-07)
3. abcac2e19fa (ref-filter.c: fix a leak in get_head_description,
   2022-09-25)
4. 962dd7ebc3e (wt-status: introduce wt_status_state_free_buffers(),
   2020-09-27).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>