Allow use of commit message as tag

[ArneBachmann]

To reference revision by its name. Should be repository-global instead of branch-local

[ArneBachmann]

We cannot enforce unique tags, unless user specifies something like --tag, because we do not even enforce commit messages at all.

[ArneBachmann]

Using --tag however, would require each later commits message to be checked against all tags in all branches, and we would need to keep track of them somewhere (simple list/set suffices).