When using Git as a store, new commits created in the underlying Git
repo are only made visible by making changes on top of them (e.g by
checking them out, so a working copy commit is created on top). That's
especially confusing when creating a new repo backed by an existing
Git repo, because the commits from that repo don't show up.
This commit prepares for fixing that by adding a function for updating
heads based on git refs. Since we don't yet track git refs (or
anything similar), the function just makes sure the refs are visible
in the Jujube repo by making them (anonymous) heads.
It's annoying to have to have the Git repo and Jujube repo in separate
directories. This commit adds `jj init --git`, which creates a new
Jujube repo with an empty, bare git repo in `.jj/git/`. Hopefully the
`jj git` subcommands will eventually provide enough functionality for
working with the Git repo that the user won't have to use Git commands
directly. If they still do, they can run them from inside `.jj/git/`,
or create a new worktree based on that bare repo.
The implementation is quite straight-forward. One thing to note is
that I made `.jj/store` support relative paths to the Git repo. That's
mostly so the Jujube repo can be moved around freely.
