It's a pretty frequent request to have support for turning off
auto-tracking of new files and to have a command to manually track
them instead. This patch adds a `snapshot.auto-track` config to decide
which paths to auto-track (defaults to `all()`). It also adds a `jj
track` command to manually track the untracked paths.
This patch does not include displaying the untracked paths in `jj
status`, so for now this is probably only useful in colocated repos
where you can run `git status` to find the untracked files.
#323
