allow multiple read-only ent processes simultaneously

Currently every time `ent` checks out a worktree of its "issues database",
it uses the local branch `entomologist-data`.  This means only one
worktree can exist at a time, because each branch can only be checked
out into at most one worktree.

Some ent operations are read-only which means we could `git worktree add`
the `origin/entomologist-data` branch instead, which uses a detached HEAD
checkout, which is sufficient for read-only operations like `ent list`
and `ent show`.

This would be useful if you're sitting in `ent edit ${ISSUE_1}` and want
to look at another issue with `ent show ${ISSUE_2}` or list the issues
with `ent list`.