Better git worktree support

[rekmarks]

git worktree support appears limited at the moment, but this feature is core to my workflow (stacking or otherwise). Situation:

• I have main as default and trunk, not checked out
• feature-a checkout out in main repo
• feature-b checked out in worktree

Running gh stack cascade in feature-a:

> gh stack cascade
Cascading feature-a onto main (using fork point)...
Done in 6s 453ms
Successfully rebased and updated refs/heads/feature-a.
Cascading feature-a... ok
Cascading feature-b onto feature-a (using fork point)...
Error: git checkout: fatal: 'feature-b' is already used by worktree at '~/worktrees/feature-b'

Running gh stack cascade in the feature-b worktree works, but there's a warning:

> gh stack cascade
Warning: could not save undo state: mkdir ~/worktrees/feature-b/.git: not a directory
Cascading feature-b onto feature-a (using fork point)...
Done in 6s 387ms

Being a worktree, feature-b/.git is a text file with the content:

gitdir: <path to feature-a>

It seems like the correct way to handle this would be one of:

• cd into worktree dirs in a subshell and run commands there (ideal)
• stop cascading when encountering a branch in a worktree (less ideal, essentially the status quo but without an error)