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.

src/libostree/ostree-sepolicy.h

@@ -44,6 +44,11 @@ OstreeSePolicy* ostree_sepolicy_new_at (int            rootfs_dfd,
                                         GCancellable  *cancellable,
                                         GError       **error);
 
+_OSTREE_PUBLIC
+OstreeSePolicy* ostree_sepolicy_new_from_commit (OstreeRepo  *repo,
+                                                 const char  *rev,
+                                                 GCancellable *cancellable,
+                                                 GError     **error);
 
 _OSTREE_PUBLIC
 GFile * ostree_sepolicy_get_path (OstreeSePolicy  *self);