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/checkout.c

@@ -1082,6 +1082,7 @@ static void orphaned_commit_warning(struct commit *old_commit, struct commit *ne
 
 	/* Clean up objects used, as they will be reused. */
 	repo_clear_commit_marks(the_repository, ALL_REV_FLAGS);
+	release_revisions(&revs);
 }
 
 static int switch_branches(const struct checkout_opts *opts,