revisions API: call diff_free(&revs->pruning) in revisions_release()

revisions API: call diff_free(&revs->pruning) in revisions_release()

Call diff_free() on the "pruning" member of "struct rev_info".  Doing
so makes several tests pass under SANITIZE=leak.

This was also the last missing piece that allows us to remove the
UNLEAK() in "cmd_diff" and "cmd_diff_index", which allows us to use
those commands as a canary for general leaks in the revisions API. See
[1] for further rationale, and 886e1084d78 (builtin/: add UNLEAKs,
2017-10-01) for the commit that added the UNLEAK() there.

1. https://lore.kernel.org/git/220218.861r00ib86.gmgdl@evledraar.gmail.com/

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