hgweb: Added safety net for PATH_INFO starting with double slash.

This happens e.g. when using the following apache config:
  RewriteRule (.*) /hgwebdir/$1 [PT]
instead of the less readable (but more correct):
  RewriteRule (.*) /hgwebdir$1 [PT]

mercurial/hgweb/hgweb_mod.py

@@ -660,6 +660,8 b' class hgweb(object):'
 
                 pi = req.env.get('PATH_INFO')
                 if pi:
+                    while pi.startswith('//'):
+                        pi = pi[1:]
                     if pi.startswith(base):
                         if len(pi) > len(base):
                             base += '/'