bug#6487: using vc-annotate-find-revision-at-line for renamed/removed files

[Stefan Monnier]

> vc-annotate-find-revision-at-line knows what VC backend it needs to
> call, so it could pass it as an optional argument to vc-find-revision.

Indeed, vc-call relies on `vc-backend' which we usually should try to
avoid calling (since usually the context already tells us which backend
to use and using another one would actually be an error).


        Stefan