bug#58383: 29.0.50; Make it easier to invert vc-prepare-patches-separately

[Sean Whitton]

Hello,

On Thu 10 Nov 2022 at 08:14PM GMT, Philip Kaludercic wrote:

> Sean Whitton <spwhitton@spwhitton.name> writes:
>
>> Hello,
>>
>> On Wed 09 Nov 2022 at 05:46PM GMT, Philip Kaludercic wrote:
>>
>>> The critical edge-case here is what happens when branches are merged.
>>> Do you pick a random branch or collect all the patches.  Or do you raise
>>> an error, but then how do you detect that vc-generically.
>>
>> Typically you wouldn't want to format patches across a merge, so I would
>> suggest raising an error.
>
> And this is something I don't think can be /expressed/ using vc, because
> while I can collect a number of revisions using `previous-revision',
> there is no general way to verify if a commit is a merge commit.

Can we do that part on a VCS-by-VCS basis?  Default to just calling
previous-revision and hoping for the best, but giving vc-git.el a chance
to raise an error.

-- 
Sean Whitton