OstreeMutableTree: add _remove method

There is no API method to remove a file or subdirectory from a MutableTree
besides directly manipulating the GHashTable returned by _get_files or
_get_subdirs. This isn't possible from an introspection binding that transforms
the returned GHashTable, and may also leave the tree checksum in an invalid
state. Introduce a new method so that removing files or subdirectories is
safe, and possible from bindings.

Closes: #1724
Approved by: jlebon

src/libostree/ostree-mutable-tree.h

@@ -77,6 +77,12 @@ gboolean ostree_mutable_tree_replace_file (OstreeMutableTree *self,
                                            const char        *checksum,
                                            GError           **error);
 
+_OSTREE_PUBLIC
+gboolean ostree_mutable_tree_remove (OstreeMutableTree *self,
+                                     const char        *name,
+                                     gboolean           allow_noent,
+                                     GError           **error);
+
 _OSTREE_PUBLIC
 gboolean ostree_mutable_tree_ensure_dir (OstreeMutableTree *self,
                                          const char        *name,