>> @@ -2416,7 +2416,10 @@ vc-log-internal-common
>>        (let ((inhibit-read-only t))
>>  	;; log-view-mode used to be called with inhibit-read-only bound
>>  	;; to t, so let's keep doing it, just in case.
>> -	(vc-call-backend backend 'log-view-mode)
>> +	(vc-call-backend backend
>> +                         (if (eq type 'with-diff)
>> +                             'region-history-mode
>> +                           'log-view-mode))
>
> Can we avoid calling region-history-mode if the backend doesn't
> implement it?  Or do you plan on adding that to all backends?

A new patch avoids calling region-history-mode
if the backend doesn't implement it: