Revert "pass --git-dir, rather than changing cwd"

This reverts commit c142696c58.

It turns out it was not needed; 681313dfd4
fixed up the git dir, so setting cwd to it works ok.

But worst, this commit broke the test suite massively. I don't understand how.
git-annex get was failing. Very weirdly, git-annex find in a fresh
clone of an annex repo, during autoinit, was displaying a side message
-- but side messages are disabled when running find.

Git/Config.hs

@@ -61,15 +61,10 @@ read' repo = go repo
 	go _ = assertLocal repo $ error "internal"
 	git_config d = withCreateProcess p (git_config' p)
 	  where
-		params =
-			[ "--git-dir"
-			, fromRawFilePath d
-			, "config"
-			, "--null"
-			, "--list"
-			]
+		params = ["config", "--null", "--list"]
 		p = (proc "git" params)
-			{ env = gitEnv repo
+			{ cwd = Just (fromRawFilePath d)
+			, env = gitEnv repo
 			, std_out = CreatePipe 
 			}
 	git_config' p _ (Just hout) _ pid =