builtin/diff: free symmetric diff members

builtin/diff: free symmetric diff members

We populate a `struct symdiff` in case the user has requested a
symmetric diff. Part of this is to populate a `skip` bitmap that
indicates which commits shall be ignored in the diff. But while this
bitmap is dynamically allocated, we never free it.

Fix this by introducing and calling a new `symdiff_release()` function
that does this for us.

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