git-subtree: guess directory from repository

Takes "humanish" part of repo like git clone does.

This took some refactoring to get right. $prefix handling is now
done inside the functions implementing the commands.

Signed-off-by: Lars Buitinck <larsmans@gmail.com>
git-subtree: kill some dead TODO items

There is a push command, and git-subtree.txt contains an example of
git subtree add usage.

Signed-off-by: Lars Buitinck <larsmans@gmail.com>