This initializes a git backed repo.
* It does the same thing as `jj init --git` except that it
has a --colocated flag to explicitly specify that we want
the .git repo to be side-by-side the .jj repo in the working
directory.
* `jj init --git` will keep the current behaviour and will not
be able to create colocated git backed repos.
* Update test snapshots.
