repo-file: s/content_checksum/contents_checksum/

This is what we call it everywhere else, so just be consistent.
It also lines up with metadata_checksum better.

https://bugzilla.gnome.org/show_bug.cgi?id=707727
builtin-commit: Don't parse the parent's GVariant by hand

Instead, use OstreeRepoFile as a handle for the parent commit.
We need to add an accessor for the metadata checksum, as that
hasn't been exposed before.

https://bugzilla.gnome.org/show_bug.cgi?id=707727
repo: Drop the branch parameter from ostree_repo_commit

It's unused. Make users explicitly write a ref if they want this;
high-level convenience API will be introduced later.

https://bugzilla.gnome.org/show_bug.cgi?id=707727
repo: Make the body parameter to ostree_repo_commit optional

https://bugzilla.gnome.org/show_bug.cgi?id=707727