When parsing git configs, support all the documented ways to write true and false, including "yes", "on", "1", etc.

This change does impact git-annex config
eg "git annex config --set annex.addunlocked on"
will store "on" and new git-annex will understand that value, while
old git-annex will error:
git-annex: bad annex.addunlocked configuration in git annex config:
Parse failure: near "on"
That seems acceptable.

Not special remote configs that are only documented as =true or =false
however. Having git-annex support other values for those would break
backwards compatability when used with old versions of git-annex. And
older versions ignore invalid special remote configs.. That would not
be a good combination.

Annex/SpecialRemote/Config.hs

@@ -17,7 +17,6 @@ import Types.UUID
 import Types.ProposedAccepted
 import Types.RemoteConfig
 import Types.GitConfig
-import qualified Git.Config
 
 import qualified Data.Map as M
 import qualified Data.Set as S
@@ -242,9 +241,17 @@ yesNoParser f v fd = genParser yesno f v fd
 	yesno _ = Nothing
 
 trueFalseParser :: RemoteConfigField -> Bool -> FieldDesc -> RemoteConfigFieldParser
-trueFalseParser f v fd = genParser Git.Config.isTrueFalse f v fd
+trueFalseParser f v fd = genParser trueFalseParser' f v fd
 	(Just (ValueDesc "true or false"))
 
+-- Not using Git.Config.isTrueFalse because git supports
+-- a lot of other values for true and false in its configs,
+-- and this is not a git config and we want to avoid that mess.
+trueFalseParser' :: String -> Maybe Bool
+trueFalseParser' "true" = Just True
+trueFalseParser' "false" = Just False
+trueFalseParser' _ = Nothing
+
 genParser
 	:: Typeable t
 	=> (String -> Maybe t)