branch: fix a leak in dwim_and_setup_tracking

branch: fix a leak in dwim_and_setup_tracking

In e89f151db1 (branch: move --set-upstream-to behavior to
dwim_and_setup_tracking(), 2022-01-28) the string returned by
dwim_branch_start() was not freed, resulting in a memory leak.

It can be reviewed with:

   $ git remote add local .
   $ git update-ref refs/remotes/local/foo HEAD
   $ git branch --set-upstream-to local/foo foo

   Direct leak of 23 byte(s) in 1 object(s) allocated from:
       ... in xstrdup wrapper.c
       ... in expand_ref refs.c
       ... in repo_dwim_ref refs.c
       ... in dwim_branch_start branch.c
       ... in dwim_and_setup_tracking branch.c
       ... in cmd_branch builtin/branch.c
       ... in run_builtin git.c

Let's free it now.

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