And add a change log.

We don't need to set the `agefile` config value because
"info/web/last-modified" is already CGit's default value for the
setting.

Reflectub is designed to work specifically with CGit, but could work
with other Git web frontends. Make this more explicit in the
description.

In order for CGit to know that the repository uses a default branch that
isn't "master", we need to set the `defbranch` setting in 'cgitrc'.

The mtime is read from either the "master" ref or the ref specified with
`defbranch`:
https://git.zx2c4.com/cgit/tree/ui-repolist.c?id=5258c297ba6fb604ae1415fbc19a3fe42457e49e#n56

I need to add another line to the repo-local cgitrc file to set the
default branch. Move this code to a function so we can reuse it.

If the default branch on GitHub changed, change the local mirror's HEAD
to match the new default.

Need to store the default branch in the database now so we can find out
whether it changed.

The default branch after mirroring was typically 'master'. On GitHub,
the default branch may not necessarily be 'master'. Change the default
branch by changing the HEAD to GitHub's default branch so that the
mirrored repository better matches GitHub.

We'll also need to make a change to the update function in case the
default branch changes after mirroring.

Now that we're done working out empty repository handling for setting
the mtime, revert this hard-coded test repository change.