Fix issues in milestone.sh swift support

Fix issues found while releasing Swift 2.0.0 using the new milestone.sh.
It was failing is various ways where scripts were expecting intermediary
milestones, and there was also a typo in a bash test and missing --tags
restriction on a git show-ref call.

I shall really rewite this with unit tests and all. One day.

Change-Id: I039fefbebd3a6b5888a42f7cb8651b31a958a865