configitems: register the 'commands.show.aliasprefix' config

hgext/show.py

@@ -51,6 +51,13 b" testedwith = 'ships-with-hg-core'"
 
 cmdtable = {}
 command = registrar.command(cmdtable)
+
+configtable = {}
+configitem = registrar.configitem(configtable)
+configitem('commands', 'show.aliasprefix',
+    default=list,
+)
+
 revsetpredicate = registrar.revsetpredicate()
 
 class showcmdfunc(registrar._funcregistrarbase):