repo/commit: Split up metadata/content commit paths

repo/commit: Split up metadata/content commit paths

There was a lot of conditionals inside `write_object()` differentating
between metadata/content, and then for content, on the different repo
types.  Further, in the metadata path since the logic is simpler, can
present a non-streaming API, and further use `OtTmpfile`, etc.

Splitting them up helps drop a lot of conditionals. We introduce a small
`CleanupUnlinkat` that allows us to fully convert to the new code style in both
functions.

This itself is still prep for fully switching to `GLnxTmpfile`.

Closes: #881
Approved by: jlebon