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