Handle worktrees in subdirectories properly

[hakoerber]

Currently, the following behaviour is quite weird:

$ grm wt add x/branch

This creates a branch called branch (unexpected) inside the directory x/branch (expected). It's also weird, because the following will fail:

$ grm wt add x/branch
$ grm wt delete x/branch
[✘] Branch "branch" is checked out in worktree, this does not look correct

The docs say:

The branch inside the worktree is always the same as the directory name of the worktree.

So the branch name should be x/branch as well. Note that the worktree cannot contain slashes, so this may be a bit ugly to set up.

[hakoerber]

Fixed with worktree rework