Merge branch 'cs/subtree-split-fixes'

An earlier attempt to optimize "git subtree" discarded too much
relevant histories, which has been corrected.

* cs/subtree-split-fixes:
  contrib/subtree: process out-of-prefix subtrees
  contrib/subtree: test history depth
  contrib/subtree: capture additional test-cases

diff --cc contrib/subtree/git-subtree.sh
Simple merge

diff --cc contrib/subtree/t/t7900-subtree.sh
Simple merge