checkout: clarify memory ownership in `unique_tracking_name()`

checkout: clarify memory ownership in `unique_tracking_name()`

The function `unique_tracking_name()` returns an allocated string, but
does not clearly indicate this because its return type is `const char *`
instead of `char *`. This has led to various callsites where we never
free its returned memory at all, which causes memory leaks.

Plug those leaks and mark now-passing tests as leak free.

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