branch: fix a leak in cmd_branch

branch: fix a leak in cmd_branch

In 98e7ab6d42 (for-each-ref: delay parsing of --sort=<atom> options,
2021-10-20) a new string_list was introduced to accumulate any
"branch.sort" setting.

That string_list is cleared in ref_sorting_options(), which is only
called when processing the "--list" sub-command.  Therefore, with other
sub-command, while having any sort option set, a leak is produced, e.g.:

   $ git config branch.sort invalid_sort_option
   $ git branch --edit-description

   Direct leak of 384 byte(s) in 1 object(s) allocated from:
       ... in xrealloc wrapper.c
       ... in string_list_append_nodup string-list.c
       ... in string_list_append string-list.c
       ... in git_branch_config builtin/branch.c
       ... in configset_iter config.c
       ... in repo_config config.c
       ... in git_config config.c
       ... in cmd_branch builtin/branch.c
       ... in run_builtin git.c

   Indirect leak of 20 byte(s) in 1 object(s) allocated from:
       ... in xstrdup wrapper.c
       ... in string_list_append string-list.c
       ... in git_branch_config builtin/branch.c
       ... in configset_iter config.c
       ... in repo_config config.c
       ... in git_config config.c
       ... in cmd_branch builtin/branch.c
       ... in run_builtin git.c

We don't have a common clean-up section in cmd_branch().  To avoid
refactoring, keep the fix simple, and while we find a better solution
which hopefuly will avoid entirely that string_list, when no sort
options are needed; let's squelch the leak sanitizer using UNLEAK().

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>