--- a/mercurial/configitems.py	Fri Jun 30 03:45:48 2017 +0200
+++ b/mercurial/configitems.py	Fri Jun 30 03:45:49 2017 +0200
@@ -631,6 +631,9 @@
 coreconfigitem('web', 'port',
     default=8000,
 )
+coreconfigitem('web', 'prefix',
+    default='',
+)
 coreconfigitem('worker', 'backgroundclose',
     default=dynamicdefault,
 )