context: clarify the various mode in the _copies property cache

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/D6933

mercurial/context.py

@@ -490,14 +490,22 b' class changectx(basectx):'
         # In compatibility mode, we return copy data from the changeset if
         # it was recorded there, and otherwise we fall back to getting it from
         # the filelogs (below).
-        if (source == 'changeset-only' or
-            (source == 'compatibility' and p1copies is not None)):
-            return p1copies or {}, p2copies or {}
-
-        # Otherwise (config said to read only from filelog, or we are in
-        # compatiblity mode and there is not data in the changeset), we get
-        # the copy metadata from the filelogs.
-        return super(changectx, self)._copies
+        if source == 'changeset-only':
+            if p1copies is None:
+                p1copies = {}
+            if p2copies is None:
+                p2copies = {}
+        elif source == 'compatibility':
+            if p1copies is None:
+                # we are in compatiblity mode and there is not data in the
+                # changeset), we get the copy metadata from the filelogs.
+                p1copies, p2copies = super(changectx, self)._copies
+        else:
+            # config said to read only from filelog, we get the copy metadata
+            # from the filelogs.
+            p1copies, p2copies = super(changectx, self)._copies
+        return p1copies, p2copies
+
     def description(self):
         return self._changeset.description
     def branch(self):