repo/commit: Dedup metadata writing API implementations

repo/commit: Dedup metadata writing API implementations

First, the streaming metadata API is pretty dumb, since metadata
should be small.  Really we should have supported a `GBytes`
version.  Currently, this API *is* used when we do local pulls,
so this commit has test coverage.  However, I plan to change
the object import to avoid using this.  But that's fine, since
I can't think of why someone would use this API.

Next, the only difference between `ostree_repo_write_metadata()` and
`ostree_repo_write_metadata_trusted()` is whether or not we pass
an output checksum; so just dedup the implementations.

Also while I'm here break out the input length validation and do
it early in the streaming case.

Closes: #881
Approved by: jlebon