Subtree remote's git-tfs ID is now applied when pulling new changesets, TfsWriter now handles determining whether to write with subtree or its owner.

This facilitates using git-tfs after having performed a subtree-split.  In the subtree's artificial revision history, the owner's git-tfs ID will not be present, so the TfsWriter will proceed as though the subtree remote were a normal remote.

Signed-off-by: Gordon Burgett <gordon.burgett@gmail.com>