pass --git-dir when reading git config when it was specified explicitly

Let GIT_DIR and --git-dir override git's protection against operating in a
repository owned by another user.

This is the same behavior other git commands have.

Sponsored-by: Jarkko Kniivilä on Patreon

CHANGELOG

@@ -38,6 +38,8 @@ git-annex (10.20220823) UNRELEASED; urgency=medium
   * Run annex.freezecontent-command and annex.thawcontent-command
     when on a crippled filesystem.
     Thanks, Reiko Asakura
+  * Let GIT_DIR and --git-dir override git's protection against operating
+    in a repository owned by another user.
 
  -- Joey Hess <id@joeyh.name>  Mon, 29 Aug 2022 15:03:04 -0400
 

Git/Config.hs

@@ -57,12 +57,20 @@ reRead r = read' $ r
 read' :: Repo -> IO Repo
 read' repo = go repo
   where
-	go Repo { location = Local { gitdir = d } } = git_config d
-	go Repo { location = LocalUnknown d } = git_config d
+	go Repo { location = Local { gitdir = d } } = git_config True d
+	go Repo { location = LocalUnknown d } = git_config False d
 	go _ = assertLocal repo $ error "internal"
-	git_config d = withCreateProcess p (git_config' p)
+	git_config isgitdir d = withCreateProcess p (git_config' p)
 	  where
-		params = ["config", "--null", "--list"]
+		params = 
+			-- Passing --git-dir changes git's behavior
+			-- when run in a repository belonging to another
+			-- user. When a gitdir is known, pass that in order
+			-- to get the local git config.
+			(if isgitdir && gitDirSpecifiedExplicitly repo
+				then ["--git-dir=."]
+				else [])
+			++ ["config", "--null", "--list"]
 		p = (proc "git" params)
 			{ cwd = Just (fromRawFilePath d)
 			, env = gitEnv repo

Git/CurrentRepo.hs

@@ -79,11 +79,11 @@ get = do
 			{ gitdir = absd
 			, worktree = Just curr
 			}
-		r <- Git.Config.read $ newFrom loc
-		let r' = r { gitDirSpecifiedExplicitly = True }
-		return $ if Git.Config.isBare r'
-			then r' { location = (location r) { worktree = Nothing } }
-			else r'
+		r <- Git.Config.read $ (newFrom loc)
+			{ gitDirSpecifiedExplicitly = True }
+		return $ if Git.Config.isBare r
+			then r { location = (location r) { worktree = Nothing } }
+			else r
 	configure Nothing Nothing = giveup "Not in a git repository."
 
 	addworktree w r = changelocation r $ Local