init: make --manifest-url flag optional

Since the --manifest-url flag is always required when creating a new
checkout, allow the url to be specified via a positional argument.
This brings it a little closer to the `git clone` UI.

Change-Id: Iaf18e794ae2fa38b20579243d067205cae5fae2f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297322


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>