linux-libre.updateScript: don't update if unchanged

The revision applies to the whole SVN tree, not just the scripts
directory, so the revision will increment sometimes with no change in
content.

pkgs/os-specific/linux/kernel/update-libre.sh

@@ -6,6 +6,7 @@ nixpkgs="$(git rev-parse --show-toplevel)"
 path="$nixpkgs/pkgs/os-specific/linux/kernel/linux-libre.nix"
 
 old_rev="$(grep -o 'rev = ".*"' "$path" | awk -F'"' '{print $2}')"
+old_sha256="$(grep -o 'sha256 = ".*"' "$path" | awk -F'"' '{print $2}')"
 
 svn_url=https://www.fsfla.org/svn/fsfla/software/linux-libre/releases/branches/
 rev="$(curl -s "$svn_url" | grep -Em 1 -o 'Revision [0-9]+' | awk '{print $2}')"
@@ -17,6 +18,11 @@ fi
 
 sha256="$(QUIET=1 nix-prefetch-svn "$svn_url" "$rev" | tail -1)"
 
+if [ "$old_sha256" = "$sha256" ]; then
+    echo "No updates for linux-libre"
+    exit 0
+fi
+
 sed -i -e "s/rev = \".*\"/rev = \"$rev\"/" \
     -e "s/sha256 = \".*\"/sha256 = \"$sha256\"/" "$path"