revisions API users: add straightforward release_revisions()

Add a release_revisions() to various users of "struct rev_list" in
those straightforward cases where we only need to add the
release_revisions() call to the end of a block, and don't need to
e.g. refactor anything to use a "goto cleanup" pattern.

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

builtin/reflog.c

@@ -248,6 +248,7 @@ static int cmd_reflog_expire(int argc, const char **argv, const char *prefix)
 		if (verbose)
 			printf(_("Marking reachable objects..."));
 		mark_reachable_objects(&revs, 0, 0, NULL);
+		release_revisions(&revs);
 		if (verbose)
 			putchar('\n');
 	}