Update source updater to support fetchgit; add aufs2 using that method. Unfortunately, AUFS2 seems to require some strict conditions on kernel options and version..

svn path=/nixpkgs/trunk/; revision=16983

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -42,6 +42,8 @@ if test -z "$finalPath"; then
       echo $tmpFile
       git checkout $rev
     fi
+    # Allow doing additional processing before .git removal
+    eval "$NIX_PREFETCH_GIT_CHECKOUT_HOOK"
     find $tmpFile -name .git\* | xargs rm -rf
 
     # Compute the hash.