win32text: use 'tiprev' when appropriate

This is cleaner than the current 'len(repo) - 1' form.

hgext/win32text.py

@@ -139,7 +139,7 b' def forbidnewline(ui, repo, hooktype, no'
     # changegroup that contains an unacceptable commit followed later
     # by a commit that fixes the problem.
     tip = repo['tip']
-    for rev in xrange(len(repo) - 1, repo[node].rev() - 1, -1):
+    for rev in xrange(repo.changelog.tiprev(), repo[node].rev() - 1, -1):
         c = repo[rev]
         for f in c.files():
             if f in seen or f not in tip or f not in c: