>>> Maybe I should express this wish in a separate bug, but:
>>>
>>> It would be nice if vc-create-tag allowed to specify start-point
>>> conveniently:
>>>
>>> git checkout -b new-branch-name START-POINT
>>>
>>> Currently I do this in shell.
>>
>> Perhaps we could do that with 'C-u C-u'.
>>
>> Yes, a separate feature request is preferable.
>
> I'm also thinking of a separate command for branch creation, wrote all
> that in bug#50643.  Thanks.

The following patch adds two new branch commands.

> However, I don't use tags much (besides creation - only "git fetch
> --tags"), so I cannot add anything meaningful here.
> Otherwise, the plan for C-x v b prefix looks great.
> Other commands, like maybe "C-x v b r" - rename, could be added then.

I can't find the existing vc API method for renaming tags/branches.

> The command could have a mode for specifying START-POINT, so for Git the
> command becomes "git checkout -b new-branch-name START-POINT".  This
> could be on C-u (unless there're other frequent "customization" cases).

The existing API method has no argument for START-POINT.
Maybe every backend could handle its prefix arg directly
from current-prefix-arg?  For example,

 (defun vc-git-create-tag (dir name branchp)
   (if current-prefix-arg (completion-read "Start point: ") ...