builtin/merge: release output buffer after performing merge

The `obuf` member of `struct merge_options` is used to buffer output in some cases. In order to not discard its allocated memory we only release its contents in `merge_finalize()` when we're not currently recursing into a subtree. This results in some situations where we seemingly do not release the buffer reliably. We thus have calls to `strbuf_release()` for this buffer scattered across the codebase. But we're missing one callsite in git-merge(1), which causes a memory leak. We should ideally refactor this interface so that callers don't have to know about any such internals. But for now, paper over the issue by adding one more `strbuf_release()` call. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>