config: register the 'devel.servercafile' config

mercurial/configitems.py

@@ -71,6 +71,9 b" coreconfigitem('devel', 'check-relroot',"
 coreconfigitem('devel', 'disableloaddefaultcerts',
     default=False,
 )
+coreconfigitem('devel', 'servercafile',
+    default='',
+)
 coreconfigitem('patch', 'fuzz',
     default=2,
 )