Re: Merge-base alias for git vc-diff

[Juri Linkov <juri@linkov.net>]

>> Since there is no good solution to reuse the existing command,
>> bug#33950 implements two new commands:
>>
>> * vc-log-mergebase (bound to ‘C-x v M L’) shows the same output as
>>   GitLab shows in the “Commits” tab of the merge request.
>>
>> * vc-diff-mergebase (bound to ‘C-x v M D’) shows the same output as
>>   GitLab shows in the “Changes” tab of the merge request.
>
> I must say I find this solution very unsatisfactory.

The reason I proposed separate commands is because I can't find
a good solution how to combine this with the existing commands.
Unsolved problems:

1. I don't know how to change the *Completions* buffer header from
   “Possible completions are:” to
   “Possible completions are (<REV>... means diff with common ancestor):”

2. How to support inserting a completion from the *Completions* buffer
   into the minibuffer but not exiting the minibuffer after choose-completion.
   Then after selecting a completion the user could add “...” to the end.

3. ‘C-u C-x v L’ asks for the limit only.  Maybe adding a new prefix arg
   ‘C-u C-u C-x v L’ could ask for two revisions: start rev and end rev.

Then ‘C-u C-u C-x v D’ could also interpret its double prefix arg as diff
with the common ancestor.