make fetchgit and nix-prefetch-git use --depth 1 to receive only a shallow clone cutting history

svn path=/nixpkgs/trunk/; revision=16047

pkgs/build-support/fetchgit/builder.sh

@@ -2,7 +2,7 @@ source $stdenv/setup
 
 header "exporting $url (rev $rev) into $out"
 
-git clone "$url" $out
+git clone --depth 1 "$url" $out
 if test -n "$rev"; then
   cd $out
   git checkout $rev

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

@@ -36,7 +36,7 @@ if test -z "$finalPath"; then
     trap "rm -rf $tmpPath" EXIT
 
     # Perform the checkout.
-    git clone "$url" $tmpFile
+    git clone --depth 1 "$url" $tmpFile
     if test -n "$rev"; then
       cd $tmpFile
       echo $tmpFile