mk/package.sh: Avoid using wc(1).

The test for empty output can be done directly using bash.

Author: briansmith

mk/package.sh

@@ -4,7 +4,7 @@ IFS=$'\n\t'
 
 # Make sure the current tree isn't dirty.
 # https://stackoverflow.com/a/5737794
-if [[ $(git status --porcelain | wc -c) -ne 0 ]]; then
+if [[ -n "$(git status --porcelain)" ]]; then
   echo Repository is dirty.
   exit 1
 fi