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>