made parentDir return a Maybe FilePath; removed most uses of it

parentDir is less safe than takeDirectory, especially when working
with relative FilePaths. It's really only useful in loops that
want to terminate at /

This commit was sponsored by Audric SCHILTKNECHT.

Git/Construct.hs

@@ -46,8 +46,8 @@ fromCwd = getCurrentDirectory >>= seekUp
 		r <- checkForRepo dir
 		case r of
 			Nothing -> case parentDir dir of
-				"" -> return Nothing
-				d -> seekUp d
+				Nothing -> return Nothing
+				Just d -> seekUp d
 			Just loc -> Just <$> newFrom loc
 
 {- Local Repo constructor, accepts a relative or absolute path. -}