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

[Philip Kaludercic]

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.