bug#57807: 29.0.50; Make vc-print-branch-log able to log multiple branches

[Juri Linkov]

>>>> Is this equivalent to `M-& git log origin/scratch/posix-spawn 4b2ca6bfc0`,
>>>> then enabling vc-git-log-view-mode in the output buffer?
>>>
>>> It's not, because vc-git-log-view-mode requires you pass four or five
>>> options to git-log to get output it can properly display.  That
>>> knowledge is locked up in vc-git.el right now.
>>
>> This part is clear, but I also wanted to know what git command
>> corresponds to this feature.  Is it just `git log` with two
>> branch names on the command line?  What additional options
>> are required?  Maybe `--graph --oneline`?  Also I see that
>> git documentation mentions `--branches[=<pattern>]`.
>
> It's something along these lines:
>
> git log -100 --graph --decorate --date=short \
>     --pretty="tformat:%d%h..: %an %ad %s" --abbrev-commit \
>     BRANCH1 BRANCH2

This is what I proposed a long ago, maybe it makes sense here.
After invoking a vc command with a prefix argument it could pop up
the minibuffer with the command line that you can edit to add or delete
more arguments for the git command.  So e.g. after 'C-u C-x v b l BRANCH1 RET'
it will insert in the minibuffer the default command `git log ... BRANCH1`,
where you can add BRANCH2 and modify any other option.  Then it runs
this command and displays the output in the vc-git-log-view-mode buffer.