lib: Add an API to construct a MutableTree from a commit

This is nicer than having the caller parse the commit
object, or indirect via the `OstreeRepoFile*` object of the root.

Will be used in ostree-rs-ext around tar parsing.

src/libostree/ostree-mutable-tree.h

@@ -52,6 +52,12 @@ GType   ostree_mutable_tree_get_type (void) G_GNUC_CONST;
 _OSTREE_PUBLIC
 OstreeMutableTree *ostree_mutable_tree_new (void);
 
+_OSTREE_PUBLIC
+OstreeMutableTree *
+ostree_mutable_tree_new_from_commit (OstreeRepo *repo,
+                                     const char *rev,
+                                     GError    **error);
+
 _OSTREE_PUBLIC
 OstreeMutableTree * ostree_mutable_tree_new_from_checksum (OstreeRepo *repo,
                                                            const char *contents_checksum,