streamclone: add explicit check for empty local repo

Stream clone doesn't work with non-empty local repositories. In upcoming
patches, we'll move stream cloning to the regular pull code path. Add an
explicit check on the repository being empty to prevent streaming clones
to non-empty repos.

mercurial/streamclone.py

@@ -25,6 +25,10 b' def canperformstreamclone(repo, remote, '
     a set of repo requirements from the remote, or ``None`` if stream clone
     isn't supported.
     """
+    # Streaming clone only works on empty repositories.
+    if len(repo):
+        return False, None
+
     # Streaming clone only works if all data is being requested.
     if heads:
         return False, None