X.Prompt: Properly handle one completion in hlComplete

This builds upon the idea of 780360abf078d47908c446941bc0c0bae6b8664d
but it fixes the issue in a bit of a nicer way; instead of throwing an
exception and closing the prompt, we simply check if there is only a
single completion available inside `hlComplete` directly.

While what 780360abf078d47908c446941bc0c0bae6b8664d said is true insofar
that we have incomplete information when inside `hlComplete`, this does
not actually matter in this case.  We have access to the complete user
input (possibly consisting of things not from this round of completion),
which is enough here.

XMonad/Prompt.hs

@@ -410,10 +410,9 @@ highlightedItem st' completions = case complWinDim st' of
       (_,_,_,_,xx,yy) = winDim
       complMatrix = splitInSubListsAt (length yy) (take (length xx * length yy) completions)
       (col_index,row_index) = complIndex st'
-    in case length completions of
-      0 -> Nothing
-      1 -> Just $ complMatrix !! col_index !! row_index
-      _ -> complMatrix !? col_index >>= (!? row_index)
+    in case completions of
+      [] -> Nothing
+      _  -> complMatrix !? col_index >>= (!? row_index)
  where
   -- | Safe version of '(!!)'.
   (!?) :: [a] -> Int -> Maybe a
@@ -767,7 +766,12 @@ handleCompletion cs = do
         hlComplete prevCompl l st =
           if | -- The current suggestion matches the command and is a
                -- proper suffix of the last suggestion, so replace it.
-               isSuffixOfCmd && isProperSuffixOfLast -> replaceCompletion
+               isSuffixOfCmd && isProperSuffixOfLast -> replaceCompletion prevCompl
+
+             | -- We only have one suggestion, so we need to be a little
+               -- bit smart in order to avoid a loop.
+               length cs == 1 ->
+                 if command st == hlCompl then put st else replaceCompletion (head cs)
 
                -- The current suggestion matches the command, so advance
                -- to the next completion and try again.
@@ -778,7 +782,7 @@ handleCompletion cs = do
 
                -- If nothing matches at all, delete the suggestion and
                -- highlight the next one.
-             | otherwise -> replaceCompletion
+             | otherwise -> replaceCompletion prevCompl
          where
           hlCompl     :: String       = fromMaybe (command st) $ highlightedItem st l
           complIndex' :: (Int, Int)   = nextComplIndex st (length l)
@@ -788,9 +792,9 @@ handleCompletion cs = do
           isProperSuffixOfLast :: Bool =      hlCompl   `isSuffixOf` prevCompl
                                       && not (prevCompl `isSuffixOf` hlCompl)
 
-          replaceCompletion :: XP () = do
+          replaceCompletion :: String -> XP () = \str -> do
               put st
-              replicateM_ (length $ words prevCompl) $ killWord Prev
+              replicateM_ (length $ words str) $ killWord Prev
               insertString' hlCompl
               endOfLine