hgweb: fix handling of arguments in the between command

The 'pairs' argument was coded to be optional, but the code would
crash if it was not provided.

mercurial/hgweb/protocol.py

@@ -62,7 +62,6 def branches(repo, req):
     yield resp
 
 def between(repo, req):
-    if 'pairs' in req.form:
         pairs = [map(bin, p.split("-"))
                  for p in req.form['pairs'][0].split(" ")]
     resp = cStringIO.StringIO()