Add dist.sh script to trivialise making a clean tarball

The source distribution for 1.0.13 (see computing/sccb#1113+) is not as pristine as it should be, so to help with future releases I propose here a new dist.sh helper script that can be run from the git repository, but clones a fresh copy of the given git reference to ensure a pristine tarball.

I hope this is welcome, happy to answer any questions. If this is not required/welcome, please disregard.