This makes each command in `handle_command()` handle its own issues
database. The commands that only need read-only access to the issues
make a detached worktree, while the commands that need read-write access
make a normal worktree with the `entomologist-data` branch checked out.
This lets us run any number of read-only operations (like `ent list`,
`ent show`, etc), even when a long-lived read-write operation (like
`ent new` or `ent edit`) is running.
Fixes `ent 317ea8ccac1d414cde55771321bdec30`.
