allow multiple read-only ent processes simultaneously

Currently every time `ent` makes a worktree to read its issues database
branch, it checks out 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 --detached` the `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`.