context: clarify the various mode in the filesadded method

The previous code was compact but a bit dense. The new proposed code deal with
each mode separately, there are some duplicated lines, but the meaning of each
mode stand out.

One of the benefit it to make it simpler to add further mode in the future.

Differential Revision: https://phab.mercurial-scm.org/D6931

mercurial/context.py

@@ -453,13 +453,20 b' class changectx(basectx):'
         modified.difference_update(self.filesadded())
         modified.difference_update(self.filesremoved())
         return sorted(modified)
+
     def filesadded(self):
         source = self._repo.ui.config('experimental', 'copies.read-from')
-        if (source == 'changeset-only' or
-            (source == 'compatibility' and
-             self._changeset.filesadded is not None)):
-            return self._changeset.filesadded or []
-        return scmutil.computechangesetfilesadded(self)
+        filesadded = self._changeset.filesadded
+        if source == 'changeset-only':
+            if filesadded is None:
+                filesadded = []
+        elif source == 'compatibility':
+            if filesadded is None:
+                filesadded = scmutil.computechangesetfilesadded(self)
+        else:
+            filesadded = scmutil.computechangesetfilesadded(self)
+        return filesadded
+
     def filesremoved(self):
         source = self._repo.ui.config('experimental', 'copies.read-from')
         if (source == 'changeset-only' or