copies: inline _backwardrenames() in pathcopies()

I'll add another filtering step in `patchcopies()` next. I need access
to the forward copies for that.

Differential Revision: https://phab.mercurial-scm.org/D10119

mercurial/copies.py

@@ -761,9 +761,12 b' def pathcopies(x, y, match=None):'
         base = None
         if a.rev() != nullrev:
             base = x
+        x_copies = _forwardcopies(a, x)
+        y_copies = _forwardcopies(a, y, base, match=match)
+        x_backward_renames = _reverse_renames(x_copies, x, match)
         copies = _chain(
-            _backwardrenames(x, a, match=match),
-            _forwardcopies(a, y, base, match=match),
+            x_backward_renames,
+            y_copies,
         )
     _filter(x, y, copies)
     return copies