perf: optimize the WorkingCopy.IsChanged() method (#1418)

* There's no need to populate a Dictionary just to diff the the "old" and "cur" Lists of Changes.
* If the two lists are of equal length and no change has occurred, we can assume that they are also reported in equal sort-order (by git-status).
* Thus, we only need to compare the two items at each successive index.

Author: goran-w

src/ViewModels/WorkingCopy.cs

@@ -1778,16 +1778,11 @@ private bool IsChanged(List<Models.Change> old, List<Models.Change> cur)
             if (old.Count != cur.Count)
                 return true;
 
-            var oldMap = new Dictionary<string, Models.Change>();
-            foreach (var c in old)
-                oldMap.Add(c.Path, c);
-
-            foreach (var c in cur)
+            for (int idx = 0; idx < old.Count; idx++)
             {
-                if (!oldMap.TryGetValue(c.Path, out var o))
-                    return true;
-
-                if (o.Index != c.Index || o.WorkTree != c.WorkTree)
+                var o = old[idx];
+                var c = cur[idx];
+                if (o.Path != c.Path || o.Index != c.Index || o.WorkTree != c.WorkTree)
                     return true;
             }