fix(scripts): stop release script failing in diff size logic

There were two problems with the logic to conditionally warn about the
size of the diff in the private repo PR, introduced in 11f41bfb1:

* It used the return code rather than the result string from the `expr`
  command. This meant the positive branch of the test was never entered.

* It didn't trim whitespace from the `wc -l` result.

This change fixes both problems.

release.sh

@@ -365,9 +365,8 @@ echo "  https://github.com/mozilla/fxa/compare/$TRAIN_BRANCH?expand=1"
 echo "  https://github.com/mozilla/fxa-private/compare/$PRIVATE_BRANCH?expand=1"
 echo
 
-PRIVATE_DIFF_SIZE=`git diff "$PRIVATE_BRANCH..$PRIVATE_DIFF_FROM" | wc -l`
-expr "$PRIVATE_DIFF_SIZE" \> 300 > /dev/null
-IS_BIG_RELEASE="$?"
+PRIVATE_DIFF_SIZE=`git diff "$PRIVATE_BRANCH..$PRIVATE_DIFF_FROM" | wc -l | awk '{$1=$1};1'`
+IS_BIG_RELEASE=`expr "$PRIVATE_DIFF_SIZE" \> 300`
 if [ "$IS_BIG_RELEASE" = "1" ]; then
   echo "The diff for the private release is pretty big. You may want to add the following comment to that PR:"
   echo