builtin/worktree: fix leaking derived branch names

builtin/worktree: fix leaking derived branch names

There are several heuristics that git-worktree(1) uses to derive the
name of the newly created branch when not given explicitly. These
heuristics all allocate a new string, but we only end up freeing that
string in a subset of cases.

Fix the remaining cases where we didn't yet free the derived branch
names. While at it, also free `opt_track`, which is being populated via
an `OPT_PASSTHRU()`.

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