ensure that gitdir is absolute

calcGitLink turns out to need it to be absolute, and it normally is,
but not if it's read from a .git file in a submodule, or perhaps from
GIT_DIR.

I should look into dropping this invariant.

Git/Construct.hs

@@ -227,7 +227,8 @@ checkForRepo dir =
 				catchDefaultIO "" (readFile $ dir </> ".git")
 			return $ if gitdirprefix `isPrefixOf` c
 				then Just $ Local 
-					{ gitdir = drop (length gitdirprefix) c
+					{ gitdir = absPathFrom dir $
+						drop (length gitdirprefix) c
 					, worktree = Just dir
 					}
 				else Nothing

Git/CurrentRepo.hs

@@ -51,8 +51,11 @@ get = do
 		configure (Just d) r = do
 			r' <- Git.Config.read r
 			-- Let GIT_DIR override the default gitdir.
-			return $ changelocation r' $
+			absd <- absPath d
-				Local { gitdir = d, worktree = worktree (location r') }
+			return $ changelocation r' $ Local
+				{ gitdir = absd
+				, worktree = worktree (location r')
+				}
 		addworktree w r = changelocation r $
 			Local { gitdir = gitdir (location r), worktree = w }
 		changelocation r l = r { location = l }