pass --no-prefix to git-diff

Otherwise we wouldn't know whether there is a prefix to be stripped.
Users may, or may not, set the `diff.noprefix' Git variable.
magit-wash-diff: get file names from various diff headers

To unambiguously determine the "source" and "destination" file
names we have to parse several header lines, and then, depending
on the diff "type", use the appropriate values.