We've discussed three options for how to store tags:

1. Each tag is a line in the file "${ISSUE}/tags".  This is what we
   use now.

   This one is bad because git conflicts are inevitable when people
   add/remove tags near each other.  This problem is what's prompting
   this whole issue.

2. Each tag is a file in the directory "${ISSUE}/tags/", with characters
   that are illegal in filenames (notably '/') escaped.  For example,
   the tag "tui/v0.1" would live in "${ISSUE}/tags/tui,1v0.1".

   This one looks a bit weird.

3. Each tag is a file somewhere under the directory "${ISSUE}/tags/",
   and tags that include the character '/' live in subdirectories
   matching the tag name.  For example, the tag "tui/v0.1" would live in
   "${ISSUE}/tags/tui/v0.1".

   I like this option but i'm a bit unsure of how some aspects would
   work...

How should entomologist handle "${ISSUE}/tags/tui/v0.1"?  Does this
issue have one tag ("tui/v0.1") or two tags ("tui" and "tui/v0.1")?

If a user runs `ent tag ${ISSUE} tui/v0.1`, i think it would be a bit
surprising if the issue now has both "tui/v0.1" like they asked for,
and also "tui" which they didn't ask for.  But maybe that's just how
hierarchical tags work?

What happens if the user runs "ent tag ${ISSUE} tui/v0.1", then "ent
tag ${ISSUE} -tui" to remove "tui"?  Does entomologist refuse, or does
it also remove "tui/v0.1"?  Probably it refuses, again because that's
how hierarchical tags work - you (obviously?) can't have "tui/v0.1"
without also having "tui".

What happens if the user runs "ent tag ${ISSUE} tui/v0.1", then runs
"ent tag ${ISSUE} -tui/v0.1"?  Let's assume "v0.1" is the only file in
the "tags/tui/" directory.  Probably we remove the "tags/tui" directory,
and now the issue has no tags?  Again a bit surprising imo.

If a user adds a tag "tui", it creates the file "${ISSUE}/tags/tui".
If the user then adds "tui/v0.1" entomologist will have to delete the
"tui" file and replace it with a "tui/" directory, then add a "v0.1"
file to that directory, but that's no big deal.

If one user adds the tag "tui", and another user adds the tag(s)
"tui/v0.1", there will be a git conflict on sync.