New function: vc-ediff

[Christoph Scholtes]

I started using ediff recently and was missing an easy way to compare 
the current buffer (or a file selected in vc-dir) with its latest 
revision in the repo. Basically, what vc-diff does, only using ediff 
instead.

Attached is a first cut at implementing this. Note, that this patch is 
just for review of the basic approach. I will add Changelog entries and 
(if applicable) manual documentation etc. later, if the approach has 
been reviewed.

Any comments are appreciated.

Thanks,
Christoph

vc-ediff_v1.patch
Description: Text document