ref-filter: add `ref_filter_clear()`

ref-filter: add `ref_filter_clear()`

We did not bother to clean up at all in `git branch` or `git tag`, and
`git for-each-ref` only cleans up a couple of members.

Add and call `ref_filter_clear()` when cleaning up a `struct
ref_filter`. Running this patch (without any test changes) indicates a
couple of now leak-free tests. This was found by running:

    $ make SANITIZE=leak
    $ make -C t GIT_TEST_PASSING_SANITIZE_LEAK=check GIT_TEST_OPTS=--immediate

(Note that the `reachable_from` and `unreachable_from` lists should be
cleaned as they are used. So this is just covering any case where we
might bail before running the reachability check.)

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>