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>]`.