context: clarify the various mode in the filesremoved 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/D6932

mercurial/context.py

@@ -469,11 +469,16 b' class changectx(basectx):'
 
     def filesremoved(self):
         source = self._repo.ui.config('experimental', 'copies.read-from')
-        if (source == 'changeset-only' or
-            (source == 'compatibility' and
-             self._changeset.filesremoved is not None)):
-            return self._changeset.filesremoved or []
-        return scmutil.computechangesetfilesremoved(self)
+        filesremoved = self._changeset.filesremoved
+        if source == 'changeset-only':
+            if filesremoved is None:
+                filesremoved = []
+        elif source == 'compatibility':
+            if filesremoved is None:
+                filesremoved = scmutil.computechangesetfilesremoved(self)
+        else:
+            filesremoved = scmutil.computechangesetfilesremoved(self)
+        return filesremoved
 
     @propertycache
     def _copies(self):