init: allow REPO_REV/--repo-rev to specify commits/tags

While the help/usage suggested that revisions would work, they never
actually did, and just throw confusing errors.  Now that we warn if
the checkout isn't tracking a branch, allow people to specify commits
or tags explicitly.  Hopefully our nags will be sufficient to keep
most people on the right path.

Bug: https://crbug.com/gerrit/11045
Change-Id: I6ea32c677912185f55ab20faaa23c6c0a4c483b3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259492
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>