pycompat: add xrange alias for Python 2

We assign xrange on Python 3 but not Python 2. Adding the missing
symbol on Python 2 allows us to use `pycompat.xrange` to get a
generator range function.

Differential Revision: https://phab.mercurial-scm.org/D4031

mercurial/pycompat.py

@@ -331,6 +331,7 b' if ispy3:'
 else:
     import cStringIO
 
+    xrange = xrange
     unicode = unicode
     bytechr = chr
     byterepr = repr