configitems: register the 'histedit.dropmissing' config

hgext/histedit.py

@@ -218,6 +218,9 b' configitem = registrar.configitem(config'
 configitem('histedit', 'defaultrev',
     default=configitems.dynamicdefault,
 )
+configitem('histedit', 'dropmissing',
+    default=False,
+)
 
 # Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
 # extensions which SHIP WITH MERCURIAL. Non-mainline extensions should