init: merge subcmd & wrapper parsers

These are manually kept in sync which is a pain.  Have the init
subcmd reuse the wrapper code directly.

Change-Id: Ica73211422c64377bacc9bb3b1d1a8d9d5f7f4ca
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/302762


Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>