Fix the use of head in the patch-tool script

Running ./scripts/patch-tool unpack hgeometry is special insofar as ls -d hgeometry* returns not one but two results:

hgeometry-0.14
hgeometry-combinatorial-0.14

This changes the subsequent call of head to print only the first line.

scripts/patch-tool

@@ -83,7 +83,7 @@ unpack_pkg() {
     local pkg=$1
     pushd packages
     cabal unpack --pristine $pkg
-    local pkg_dir="$(ls -d $pkg* | head)"
+    local pkg_dir="$(ls -d $pkg* | head -n 1)"
     if [ -z "$pkg_dir" ]; then
         echo "failed to unpack $pkg"
         exit 1