log: fix a memory leak in "git show <revision>..."

Fix a memory leak in code added in 5d7eeee2ac6 (git-show: grok blobs,
trees and tags, too, 2006-12-14). As we iterate over a "<revision>..."
command-line and encounter ad OBJ_COMMIT we want to use our "struct
rev_info", but with a "pending" array of one element: the one commit
we're showing in the loop.

To do this 5d7eeee2ac6 saved away a pointer to rev.pending.objects and
rev.pending.nr for its iteration. We'd then clobber those (and alloc)
when we needed to show an OBJ_COMMIT.

We'd therefore leak the "rev.pending" we started out with, and only
free the new "rev.pending" in the "OBJ_COMMIT" case arm as
prepare_revision_walk() would draw it down.

Let's fix this memory leak. Now when we encounter an OBJ_COMMIT we
save away the "rev.pending" before clearing it. We then add a single
commit to it, which our indirect invocation of prepare_revision_walk()
will remove. After that we restore the "rev.pending".

Our "rev.pending" will then get free'd by the release_revisions()
added in f6bfea0ad01 (revisions API users: use release_revisions() in
builtin/log.c, 2022-04-13)

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