py3: use range instead of xrange in tests/test-convert-mtn.t

xrange in not present on Python 3.

This change will make the test a bit slower on Python 2.

Differential Revision: https://phab.mercurial-scm.org/D2328

tests/test-convert-mtn.t

@@ -218,7 +218,7 b' test diverging directory moves'
 test large file support (> 32kB)
 
   >>> fp = open('large-file', 'wb')
-  >>> for x in xrange(10000): fp.write(b'%d\n' % x)
+  >>> for x in range(10000): fp.write(b'%d\n' % x)
   >>> fp.close()
   $ md5sum.py large-file
   5d6de8a95c3b6bf9e0ffb808ba5299c1  large-file