It seems better to have the caller pass the transaction description
when we finish the transaction than when we start it. That way we have
all the information we want to include more readily available.
Allowing `jj init --git` in an existing Git repo creates a second Git
store in `.jj/repo/store/git`, totally disconnected from the existing
Git store. This will only produce extremely confusing bugs for users,
since any operations they make in Git will *not* be reflected in the
jj repo.
If the existing git repo contains local and remote branches of the same name,
one of the remote branches is probably a tracking remote branch. Let's show
a hint how to set up tracking branches. The tracking state could be derived
from .git/config, but doing that automatically might cause another issue like
#1862, which could have been mitigated by git.auto-local-branch = false.
