subtree: validate --prefix against commit in split

subtree: validate --prefix against commit in split

git subtree split currently validates --prefix against the working tree.
This breaks when splitting an older commit or when the working tree does
not contain the subtree, even though the commit does.

For example:

  git subtree split --prefix=pkg <commit>

fails if pkg was removed later, even though it exists in <commit>.

Fix this by validating the prefix against the specified commit using
git ls-tree instead of the working tree.

Add a test to ensure this behavior does not regress.

Signed-off-by: Pushkar Singh <pushkarkumarsingh1970@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>