Propigate GIT_DIR and GIT_WORK_TREE environment to external special remotes.

Since git-annex unsets these when started, they have to be explicitly
propigated. Also, this makes --git-dir and --work-tree settings be
reflected in the environment.

The need for this came up in
https://github.com/DanielDent/git-annex-remote-rclone/issues/3

Git.hs

@@ -26,6 +26,7 @@ module Git (
 	repoDescribe,
 	repoLocation,
 	repoPath,
+	repoWorkTree,
 	localGitDir,
 	attributes,
 	attributesLocal,
@@ -73,6 +74,10 @@ repoPath Repo { location = Local { gitdir = d } } = d
 repoPath Repo { location = LocalUnknown dir } = dir
 repoPath Repo { location = Unknown } = error "unknown repoPath"
 
+repoWorkTree :: Repo -> Maybe FilePath
+repoWorkTree Repo { location = Local { worktree = Just d } } = Just d
+repoWorkTree _ = Nothing
+
 {- Path to a local repository's .git directory. -}
 localGitDir :: Repo -> FilePath
 localGitDir Repo { location = Local { gitdir = d } } = d