Teach fsck about partial commits

An OSTree user noticed that `ostree fsck` would produce `missing
object` errors in the case of interrupted pulls.

It's possible to do e.g. `ostree pull --subpath=/usr/share/rpm ...`,
which gets you just that portion of the commit.  The use case for this
was being able to see what changes would appear in an update before
actually downloading all of it.

(I think this would be better covered by static deltas, but those
 aren't final yet, and `--subpath` predates it)

Further, `.commitpartial` is used as a successor to the `transaction`
symlink for more precise knowledge in the case where a pull was
interrupted that we needed to resume scanning.

So it makes sense for `ostree fsck` to be aware of it.

doc/ostree-sections.txt

@@ -255,6 +255,7 @@ ostree_repo_write_content_finish
 ostree_repo_resolve_rev
 ostree_repo_list_refs
 ostree_repo_load_variant
+ostree_repo_load_commit
 ostree_repo_load_variant_if_exists
 ostree_repo_load_file
 ostree_repo_load_object_stream