mercurial: expose HGUSER into environ becuase some extensions explicitly rely on this.

- makes it clear it's used for Mercurial extensions.

vcsserver/http_main.py

@@ -331,6 +331,8 b' class HTTPApplication(object):'
         username = dict_conf.get('username')
         if username:
             environ['REMOTE_USER'] = username
+            # mercurial specific, some extension api rely on this
+            environ['HGUSER'] = username
 
         ip = dict_conf.get('ip')
         if ip: