bug#38044: 27.0.50; There should be an easier way to look at a specific vc commit

[Juri Linkov]

>> I rewrote the patch to make it more flexible, so now the function
>> can check 'vc-log-view-type' for the value 'with-diff'.
>
> Generally, an explicit argument is better than an implicit one.
>
> So what's the idea here? Gracefully degrading to just showing a log entry
> when the backend doesn't support the WITH-DIFF feature?

Yes, if a backend doesn't support WITH-DIFF, then do nothing special.

> (BTW, the print-revision approach seems better for graceful
> degradation, but I'm tired of arguing for that).

The print-log approach is not bad too:
'C-1 C-x v L' is mnemonic ("show me just 1 revision") and easy to type.

> I think it would work, but for that all backends need support for
> region-history-mode anyway. The caller function doesn't have a way to 
> check the lack of support.

The new patch I sent now in another message
checks if a backend supports region-history-mode.