clone: use free() instead of UNLEAK()

clone: use free() instead of UNLEAK()

Change an UNLEAK() added in 0c4542738e6 (clone: free or UNLEAK further
pointers when finished, 2021-03-14) to use a "to_free" pattern
instead. In this case the "repo" can be either this absolute_pathdup()
value, or in the "else if" branch seen in the context the the
"argv[0]" argument to "main()".

We can only free() the value in the former case, hence the "to_free"
pattern.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>