Merge branch 'jc/merge-subtree'

Merge branch 'jc/merge-subtree'

* jc/merge-subtree:
  A new merge stragety 'subtree'.

It is safe to merge this early as this is a feature that user
explicitly needs to ask for and would not trigger otherwise.  A
known issue with the current implementation is that the subtree
matching heuristics is very stupid.  It could run ls-tree twice
and try to count intersection.

Giving it wider audience would help it to get improved by
motivated volunteers.

Signed-off-by: Junio C Hamano <junkio@cox.net>