Hmm... can't actually filter rev-list on the subdir name.

Otherwise we can't keep track of parent relationships. Argh.

This change makes it "work", but we get a bunch of empty commits.

Added a --onto option, but it's so complicated I can't tell if it works.

Use information about prior splits to make sure merges work right.

Add a new --rejoin option.

The idea is to join the new split branch back into this one, so future
splits can append themselves to the old split branch. We mark the split
branch's history in our merge commit, so we can pull it back out later.

Print out the newly created commitid at the end, for use in other scripts.

We now copy the other stuff about a commit (changelog, author, etc).

'git subtree split' now basically works.

basic options parsing and whatnot.