Disallow branch names starting with a letter which is already used by any commit id

[rolandkrueger]

It is still possible to create two commits with the same id:

• create a branch, e.g. develop
• create some commits on that branch
• delete this branch
• create another branch starting with 'd', e.g. dist
• create some commits on that branch
  -> these commits get the same id as the commits from the develop branch