Setting up and maintaining git-only mirrors of v2 inboxes is
complex since multiple commands are required to clone and fetch
into epochs.
Unlike grokmirror, these commands do not require any
configuration. Instead, they rely on existing git config files
and work like "git clone --mirror" and "git fetch",
respectively.
Like grokmirror, they use manifest.js.gz, but only on a
per-inbox basis so users won't have to clone every inbox of a
large instance nor edit config files to include/exclude inboxes
they're interested in.