check once and exit instead of looping

Some users may prefer to schedule git repository checks at set times rather than checking continuously (to reduce hits on remote servers for instance). With this patch, if git-dude's check interval set to 0, the script exits after a single check. Then, using systemd timers in Linux or launchd launch agents in OS X, git-dude runs can be scheduled as desired.

git-dude

@@ -122,6 +122,9 @@ while true; do
       fi
     done
   fi
-
-  sleep $interval
+  if [ "$interval" = "0" ]; then    
+     exit 0
+  else
+     sleep $interval
+  fi 
 done