support boolean git configs that are represented by the name of the setting with no value

Eg"core.bare" is the same as "core.bare = true".

Note that git treats "core.bare =" the same as "core.bare = false", so the
code had to become more complicated in order to treat the absense of a
value differently than an empty value. Ugh.

Command/Config.hs

@@ -69,8 +69,9 @@ seek (UnsetConfig ck@(ConfigKey name)) = checkIsGlobalConfig ck $ commandAction
 seek (GetConfig ck) = checkIsGlobalConfig ck $ commandAction $
 	startingCustomOutput (ActionItemOther Nothing) $ do
 		getGlobalConfig ck >>= \case
-			Nothing -> return ()
 			Just (ConfigValue v) -> liftIO $ S8.putStrLn v
+			Just NoConfigValue -> return ()
+			Nothing -> return ()
 		next $ return True
 
 checkIsGlobalConfig :: ConfigKey -> Annex a -> Annex a