repo: Add an API to init OstreeSePolicy from commit directly

This is part of `OstreeCommitModifier`, but I'm not using
that in some of the ostree-ext Rust code.

It just makes more sense as a direct policy API, where it should
have been in the first place.  There's already support for
setting a policy object on a commit modifier, so that's all the
old API needs to do now.

apidoc/ostree-sections.txt

@@ -522,6 +522,7 @@ ostree_repo_file_get_type
 OstreeSePolicy
 ostree_sepolicy_new
 ostree_sepolicy_new_at
+ostree_sepolicy_new_from_commit
 ostree_sepolicy_get_path
 ostree_sepolicy_get_name
 ostree_sepolicy_get_label