VCS[git], CodePaster: Add support for applying patches.

Modify CodePaster::fetch to do a mimetype detection on the
content, create a filename with the matching extension and
open that file.
This gives correct syntax highlighting  and makes "Save as"
more convenient. Keep the file around and delete on exit.

Modify patch mimetype with some content detection (higher priority
than C++).

Add a "current patch file" to the VCSBasePlugin::State. Add "Apply
patch" to git with whitespace fix.