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.

Git/Construct.hs

@@ -6,7 +6,6 @@
  -}
 
 module Git.Construct (
-	fromCurrent,
 	fromCwd,
 	fromAbsPath,
 	fromPath,
@@ -21,8 +20,6 @@ module Git.Construct (
 ) where
 
 import System.Posix.User
-import System.Posix.Env (getEnv, unsetEnv)
-import System.Posix.Directory (changeWorkingDirectory)
 import qualified Data.Map as M hiding (map, split)
 import Network.URI
 
@@ -31,28 +28,6 @@ import Git.Types
 import Git
 import qualified Git.Url as Url
 
-{- Finds the current git repository.
- -
- - GIT_DIR can override the location of the .git directory.
- -
- - When GIT_WORK_TREE is set, chdir to it, so that anything using
- - this repository runs in the right location. However, this chdir is
- - done after determining GIT_DIR; git does not let GIT_WORK_TREE
- - influence the git directory.
- -
- - Both environment variables are unset, to avoid confusing other git
- - commands that also look at them. This would particularly be a problem
- - when GIT_DIR is relative and we chdir for GIT_WORK_TREE. Instead,
- - the Git module passes --work-tree and --git-dir to git commands it runs.
- -}
-fromCurrent :: IO Repo
-fromCurrent = do
-	r <- maybe fromCwd fromPath =<< getEnv "GIT_DIR"
-	maybe noop changeWorkingDirectory =<< getEnv "GIT_WORK_TREE"
-	unsetEnv "GIT_DIR"
-	unsetEnv "GIT_WORK_TREE"
-	return r
-
 {- Finds the git repository used for the Cwd, which may be in a parent
  - directory. -}
 fromCwd :: IO Repo
@@ -251,3 +226,5 @@ newFrom l = return Repo
 	, remotes = []
 	, remoteName = Nothing
 	}
+
+