fix code duplication

app/src/main/java/org/isoron/uhabits/models/RepetitionList.java

@@ -130,7 +130,7 @@ public abstract class RepetitionList
         for (Repetition r : reps)
         {
             Calendar date = DateUtils.getCalendar(r.getTimestamp());
-            int weekday = date.get(Calendar.DAY_OF_WEEK) % 7;
+            int weekday = DateUtils.getWeekday(r.getTimestamp());
             date.set(Calendar.DAY_OF_MONTH, 1);
 
             long timestamp = date.getTimeInMillis();

app/src/main/java/org/isoron/uhabits/utils/DateUtils.java

@@ -206,7 +206,7 @@ public abstract class DateUtils
     public static int getWeekday(long timestamp)
     {
         GregorianCalendar day = getCalendar(timestamp);
-        return day.get(DAY_OF_WEEK) % 7;
+        return javaWeekdayToLoopWeekday(day.get(DAY_OF_WEEK));
     }
 
     /**