Warn about unsupported core.sharedRepository=0xxx when set

This spams the user with a lot of messages, but it seems like busywork to
avoid that and only warn once, since this warning will go away when it gets
implemented.

Also fix parsing of the octal value.

Sponsored-by: Kevin Mueller on Patreon

Git/ConfigTypes.hs

@@ -1,6 +1,6 @@
 {- git config types
  -
- - Copyright 2012, 2017 Joey Hess <id@joeyh.name>
+ - Copyright 2012-2023 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -10,6 +10,7 @@
 module Git.ConfigTypes where
 
 import Data.Char
+import Numeric
 import qualified Data.ByteString.Char8 as S8
 
 import Common
@@ -31,7 +32,8 @@ getSharedRepository r =
 			"all" -> AllShared
 			"world" -> AllShared
 			"everybody" -> AllShared
-			_ -> maybe UnShared UmaskShared (readish (decodeBS v))
+			_ -> maybe UnShared UmaskShared 
+				(fmap fst $ headMaybe $ readOct $ decodeBS v)
 		Just NoConfigValue -> UnShared
 		Nothing -> UnShared