Merge branch 'ps/validate-prefix-in-subtree-split' into jch

"git subtree split --prefix=P <commit>" now checks the prefix P
against the tree of the (potentially quite different from the
current working tree) given commit.

* ps/validate-prefix-in-subtree-split:
  subtree: validate --prefix against commit in split

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

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