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

[Eli Zaretskii <eliz@gnu.org>]

> From: Sean Whitton <spwhitton@spwhitton.name>
> Date: Wed, 14 Sep 2022 10:19:48 -0700
> 
> vc-git-log-edit-mode is already able to properly understand git-log(1)
> output which logs multiple branches.  Here's a hack to demonstrate that:
> 
>     (let ((default-directory "~/src/emacs/")
>           (vc-git-log-switches
>            (cons "origin/scratch/posix-spawn" vc-git-log-switches)))
>       (call-process-shell-command "git branch -f tmp 4b2ca6bfc0")
>       (vc-print-branch-log "tmp"))
> 
> (4b2ca6bfc0 is an old tip of master close to scratch/posix-spawn.)
> 
> An example of a case in which it is useful to produce logs like this is
> when you're trying to figure out how several feature branches relate to
> each other and to the trunk.
> 
> I don't think the idea of producing a log showing more than one branch
> at once is fundamentally git-specific.  So I would like to extend
> vc-print-branch-log to allow logging more than one branch, or
> alternatively, add a vc-print-branch-logs command.  I believe that there
> are two design issues, on which I'm requesting review/input:
> 
> (1) Extending the VC history functions API to permit expressing a
> request for the backend to print a log of multiple branches.  We could
> allow the START-REVISION argument to `print-log' to take a list of
> branches, or add an additional optional argument
> ADDITIONAL-START-REVISIONS.  The latter seems more robust to me.
> 
> (2) How could vc-print-branch-log prompt the user for multiple branch
> names?  Magit has the user separate branch names with commas, offering
> completion for each branch name.  But I suppose commas never appearing
> in branch names is git-specific.  So the command could prompt the user
> repeatedly for branch names, until the user hits RET to input a
> zero-length branch name, indicating they've input them all.

I'm not sure I understand: how is this different from what "C-x v L"
already produces?