This reimplements the change 9faa4670d5 "cli: on init, import git refs prior
to importing HEAD." Initialization is special because the HEAD ref isn't
available to jj yet, and there is an empty working-copy commit.
The initialization function could be refactored to go through the common code
path, but I think doing that would make future improvement harder. We might
want to initialize tracking branches based on .git/config for example.
Fixes#2942
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.
