update-tracker.sh: handling of 'admin' directory

The new process now check if there are untracked files in the repo. If
yes, ask user if changes should be discarded, and abort if not.

Then at end of script, the user is asked if 'admin' dir should be
deleted.

This is easier to implement than a new commit in the local branch to
remove the directory, because this way we don't have to deal with
potential merge conflicts which would be difficult to handle in an
automated way.

Fixes #16345