builtin/merge: fix leaking `struct cmdnames` in `get_strategy()`

builtin/merge: fix leaking `struct cmdnames` in `get_strategy()`

In "builtin/merge.c" we use the helper infrastructure to figure out what
merge strategies there are. We never free contents of the `cmdnames`
structures though and thus leak their memory.

Fix this by exposing the already existing `clean_cmdnames()` function to
release their memory. As this name isn't quite idiomatic, rename it to
`cmdnames_release()` while at it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>