configitems: register the 'convert.git.extrakeys' config

hgext/convert/__init__.py

@@ -46,6 +46,9 b" configitem('convert', 'cvsps.mergeto',"
 configitem('convert', 'git.committeractions',
     default=lambda: ['messagedifferent'],
 )
+configitem('convert', 'git.extrakeys',
+    default=list,
+)
 
 # Commands definition was moved elsewhere to ease demandload job.