configitems: register the 'web.prefix' config

mercurial/configitems.py

@@ -631,6 +631,9 b" coreconfigitem('web', 'ipv6',"
 coreconfigitem('web', 'port',
     default=8000,
 )
+coreconfigitem('web', 'prefix',
+    default='',
+)
 coreconfigitem('worker', 'backgroundclose',
     default=dynamicdefault,
 )

mercurial/hgweb/hgwebdir_mod.py

@@ -177,7 +177,7 b' class hgwebdir(object):'
         if self.stripecount:
             self.stripecount = int(self.stripecount)
         self._baseurl = self.ui.config('web', 'baseurl')
-        prefix = self.ui.config('web', 'prefix', '')
+        prefix = self.ui.config('web', 'prefix')
         if prefix.startswith('/'):
             prefix = prefix[1:]
         if prefix.endswith('/'):

mercurial/hgweb/server.py

@@ -277,7 +277,7 b' class MercurialHTTPServer(_mixin, httpse'
 
         handler.preparehttpserver(self, ui)
 
-        prefix = ui.config('web', 'prefix', '')
+        prefix = ui.config('web', 'prefix')
         if prefix:
             prefix = '/' + prefix.strip('/')
         self.prefix = prefix