copies: show we can read data from the "first" mapping

In practice this make no difference since `t` is a copy of `a`. Having this
change before hand will make the next cleanup changeset clearer.

This is work on the road to using some native code for some performance critical
part of copy tracing.

mercurial/copies.py

@@ -67,7 +67,7 b' def _chain(a, b):'
     """chain two sets of copies 'a' and 'b'"""
     t = a.copy()
     for k, v in pycompat.iteritems(b):
-        t[k] = t.get(v, v)
+        t[k] = a.get(v, v)
     return t