Avoid Git.Config.updateLocation adding "/.git" to the end of the repo

path to a bare repo when git config is not allowed to list the configs
due to the CVE-2022-24765 fix.

That resulted in a confusing error message, and prevented the nice
message that explains how to mark the repo as safe to use.

Made isBare a tristate so that the case where core.bare is not returned can
be handled.

The handling in updateLocation is to check if the directory
contains config and objects and if so assume it's bare.
Note that if that heuristic is somehow wrong, it would construct a repo
that thinks it's bare but is not. That could cause follow-on problems,
but since git-annex then checks checkRepoConfigInaccessible, and skips
using the repo anyway, a wrong guess should not be a problem.

Sponsored-by: Luke Shumaker on Patreon

Git/Config.hs

@@ -133,14 +133,28 @@ store' k v repo = repo
  - based on the core.bare and core.worktree settings.
  -}
 updateLocation :: Repo -> IO Repo
-updateLocation r@(Repo { location = LocalUnknown d })
-	| isBare r = ifM (doesDirectoryExist (fromRawFilePath dotgit))
-			( updateLocation' r $ Local dotgit Nothing
-			, updateLocation' r $ Local d Nothing
-			)
-	| otherwise = updateLocation' r $ Local dotgit (Just d)
+updateLocation r@(Repo { location = LocalUnknown d }) = case isBare r of
+	Just True -> ifM (doesDirectoryExist (fromRawFilePath dotgit))
+		( updateLocation' r $ Local dotgit Nothing
+		, updateLocation' r $ Local d Nothing
+		)
+	Just False -> mknonbare
+	{- core.bare not in config, probably because safe.directory
+	 - did not allow reading the config -}
+	Nothing -> ifM (Git.Construct.isBareRepo (fromRawFilePath d))
+		( mkbare
+		, mknonbare
+		)
   where
 	dotgit = d P.</> ".git"
+	-- git treats eg ~/foo as a bare git repository located in
+	-- ~/foo/.git if ~/foo/.git/config has core.bare=true
+	mkbare = ifM (doesDirectoryExist (fromRawFilePath dotgit))
+		( updateLocation' r $ Local dotgit Nothing
+		, updateLocation' r $ Local d Nothing
+		)
+	mknonbare = updateLocation' r $ Local dotgit (Just d)
+
 updateLocation r@(Repo { location = l@(Local {}) }) = updateLocation' r l
 updateLocation r = return r
 
@@ -212,8 +226,9 @@ boolConfig' :: Bool -> S.ByteString
 boolConfig' True = "true"
 boolConfig' False = "false"
 
-isBare :: Repo -> Bool
-isBare r = fromMaybe False $ isTrueFalse' =<< getMaybe coreBare r
+{- Note that repoIsLocalBare is often better to use than this. -}
+isBare :: Repo -> Maybe Bool
+isBare r = isTrueFalse' =<< getMaybe coreBare r
 
 coreBare :: ConfigKey
 coreBare = "core.bare"