This commit does 4 things:
* Adds a script which will: (a) clone from a git tag, make release,
and give you very detailed instructions as to what to do from that
point.
* Changes `push-official-release.sh` so we can't push "dirty"
releases anymore (which `build-official-release.sh` also double
checks at the end.)
* Fixes #9576 by ensuring a correct umask.
* Changes common.sh to gtar all the way through, to ensure that
bloody OS X tar never touches the release process, because I don't
want to have to understand two tar programs and how release
artifacts are created from both (c.f. #10615.)