Encapsulate starting a new version into single make command

[andy-maier]

Encapsulate the starting of a new version into a single make command, e.g. VERSION=M.N.U make start

That make target should perform these steps:

• Check that the VERSION env var is set
• Determine the corresponding base branch (= head of the start version commit, i.e. master or stable_M.N)
• Check out the base branch (step 2)
• Create a dummy change fragment file so we get a commit (to separate the start tag from the last release tag).
• Create a start-version tag ${VERSION}a0 and push the tag (step 3)
• Commit and push the branch (step 4)
• Create a PR for the branch (step 6)

The "Starting a new version" docs section then becomes:

• Run VERSION=M.N.U make start
• Create a milestone ${VERSION}, if not yet existing (step 5)
• Go through open issues and set to new milestone (step 7)
• Merge PR (step 8)
• Sync and cleanup local repo (step 9)