use shallow clones for gen-parrot

gistfile1.diff

diff --git a/tools/lib/NQP/Configure.pm b/tools/lib/NQP/Configure.pm
index ac397e1..1d6e7d1 100644
--- a/tools/lib/NQP/Configure.pm
+++ b/tools/lib/NQP/Configure.pm
@@ -203,20 +203,14 @@ sub git_checkout {
 
     # get an up-to-date repository
     if (! -d $dir) {
-        system_or_die('git', 'clone', $repo, $dir);
+        system_or_die('git', 'clone', '--depth', '1', '--branch', $checkout, $repo, $dir);
         chdir($dir);
         system('git', 'config', 'remote.origin.pushurl', $pushurl)
             if defined $pushurl;
     }
     else {
         chdir($dir);
-        system_or_die('git', 'fetch');
-    }
-
-    if ($checkout) {
-        system_or_die('git', 'checkout', $checkout);
-        system_or_die('git', 'pull') 
-            if slurp('.git/HEAD') =~ /^ref:/;
+        system_or_die('git', 'fetch', 'origin', $checkout);
     }
 
     my $git_describe;