Add support for core.worktree, and fix support for GIT_WORK_TREE and GIT_DIR.

The environment needs to override git-config. Changed when git config is
read, and avoid rereading it once it's been read.

chdir for both worktree settings.

test.hs

@@ -26,7 +26,7 @@ import qualified Annex
 import qualified Annex.UUID
 import qualified Backend
 import qualified Git.Config
-import qualified Git.Construct
+import qualified Git.CurrentRepo
 import qualified Git.Filename
 import qualified Locations
 import qualified Types.Backend
@@ -721,7 +721,7 @@ git_annex_expectoutput command params expected = do
 -- are not run; this should only be used for actions that query state.
 annexeval :: Types.Annex a -> IO a
 annexeval a = do
-	s <- Annex.new =<< Git.Config.read =<< Git.Construct.fromCurrent
+	s <- Annex.new =<< Git.CurrentRepo.get
 	Annex.eval s $ do
 		Annex.setOutput Types.Messages.QuietOutput
 		a