`perf` shows that the majority of the time it takes to run "ent anything"
is spent reading the issue database, and the majority of the time it
takes to read the database is spend running `git log` to find the author
and ctime of issues and comments.

I've reducted this a bit (in the `speed-test` git branch) by running
`git log` half as many times, but the shape of the flame graph remains
the same.

We've discussed lazy-loading things to avoid doing this work when we don't
need to.  For example, if we're trying to look up the tags of issue A,
there's no need to ingest all the comments of issue B.

Another option is to store Author and Creation-time the normal way, as
files in the issue directory.  We'd never have to run `git log` at all.
I've verified that this makes ent *very* fast.