cubicweb-vcreview #1957980 use our knowledge of what's in a patch repo to not depend on file names [validation pending]

http://www.cubicweb.org/ticket/1956991 was rejected with "the patch name is not .diff so we cannot display it", which feels like a not user friendly behavior on vcreview's side : the cube manages a repository of patches, so it seems to me that we can quite safely assume that any file managed in the repository (apart from series) is a patch and thus has the text/x-diff mime type. While having the file named as "mumble_frobble.diff" is still a good practice, this enhancement would make vcreview more forgiving to forgetfulness.