init: respect --repo-rev changes

We respect this option when running the first `repo init`, but then
silently ignore it once the initial sync is done.  Make sure users
are able to change things on the fly.

We refactor the wrapper API to allow reuse between the two init's.

Bug: https://crbug.com/gerrit/11045
Change-Id: Icb89a8cddca32f39a760a6283152457810b2392d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/260032
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>