There are `entomologist-data` branches "in the wild" that have issues
with tags files, so we need to migrate those somehow.

We discussed a couple of ways to do this:

1. Have the ent library do it automatically and transparently when it
   comes across an old-style tags file.  In `Issue::new_from_dir()` we'd
   detect the old format, parse it, then immediately delete the file,
   write the new format directory, and commit the result.

   This is good:
    - It's easy for the human

   This is bad:
    - entomologist will need to carry along this crufty code until we
      think there are no more old-style repos in existence


2. Make a stand-alone tool (like the `set-done-time` tool in
   <https://git.glyphs.tech/taproot-manufacturing/entomologist/pulls/27>.
   The ent binary does *not* know how to read the old tags file, and will
   error out if it comes across an issue with such a file.  The human
   would then manually run the tags-representation updater tool.
   This tool would make the git worktree, walk the issues, find the
   old-style tags, rewrite them in the new format, and commit the result.

   This is good:
    - Keeps the core entomologist code clean and "modern".

   This is bad:
    - This tool does manual/direct manipulation of the on-disk format,
      which means it duplicates a lot of what the entomologist library
      does.