We want to move git-diff-goto-source from rc/tools/git.kak
to rc/filetype/diff.kak (or should we could create
rc/tools/diff.kak?). Either way, create the diff module so we can
formalize this dependency.
Currently this module only provides highlighters, so require it
wherever we reference them.
Keep the diff-select-{file,hunk} commands outside the module because
people might already use them in git buffers.
b84abd57de