fetchgit: add postFetch argument

pkgs/build-support/fetchgit/default.nix

@@ -15,6 +15,9 @@ in
 , fetchSubmodules ? true, deepClone ? false
 , branchName ? null
 , name ? urlToName url rev
+, # Shell code executed after the file has been fetched
+  # successfully. This can do things like check or transform the file.
+  postFetch ? ""
 }:
 
 /* NOTE:
@@ -54,7 +57,7 @@ stdenv.mkDerivation {
   outputHashMode = "recursive";
   outputHash = sha256;
 
-  inherit url rev leaveDotGit fetchSubmodules deepClone branchName;
+  inherit url rev leaveDotGit fetchSubmodules deepClone branchName postFetch;
 
   GIT_SSL_CAINFO = "${cacert}/etc/ssl/certs/ca-bundle.crt";