Generate links to function definitions

Hydra passes the full revision in to the input, which we pass through.

If we don't get this ,we try to get it from other sources, or default to
master which should have the definition in a close-ish location.

All published docs should have theURL resolve properly, only local
hackers will have the link break.

doc/shell.nix

@@ -1,5 +1,5 @@
 { pkgs ? import ../. {} }:
-(import ./default.nix).overrideAttrs (x: {
+(import ./default.nix {}).overrideAttrs (x: {
   buildInputs = x.buildInputs ++ [ pkgs.xmloscopy pkgs.ruby ];
 
 })