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.
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.
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.
repo: Make the body parameter to ostree_repo_commit optional
repo-file: Base OstreeRepoFile on trees instead of commits

We want an OstreeRepoFile to be the way to represent a filesystem
tree inside an ostree repository, so change ostree_repo_file_new_root
to take two tree IDs, and add a convenience method to replace
the commit usages.
repo: Change the pairs of checksums to instead be based on OstreeRepoFiles

We want an OstreeRepoFile to be the way to reference a "filesystem
tree" that's stored in the repo, which is a combination of a DIR_TREE
and a DIR_META. The idea is that once you write an mtree to the repo
using ostree_repo_write_mtree, it becomes serialized and you get an
OstreeRepoFile in return.

Change any APIs that care about DIR_TREE / DIR_META checksums to care
about OstreeRepoFiles instead, which right now is mostly is
ostree_repo_write_commit.
repo-libarchive: Apply commit modifiers to libarchive archives as well

And document the libarchive methods as well, so we can pass a NULL
commit modifier.
repo: Make read_commit spit out a resolved commit ref as well