init: expose --worktree option

There's a few rough edges here still, but no known corruption ones,
so open it up a bit for people to experiment with.

Bug: https://crbug.com/gerrit/11486
Change-Id: I81e0122ab6d3e032c546c8239dd4f03740676e80
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/299242


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>