Comment 3 for bug 262261

So there *is* Tree.get_file_mtime() and for RevisionTrees it is defined as:
    def get_file_mtime(self, file_id, path=None):
        ie = self._inventory[file_id]
        revision = self._repository.get_revision(ie.revision)
        return revision.timestamp

^- Notice that this deserializes a new Revision object for every request, and does *no* caching. So for 10,000 files in a tree, it does 10,000 deserializations.

Which is probably why we don't use it at present.