nofib-compare: Fix header of Markdown output

I noticed that the current output is all garbage, e.g. benchmark names (row headers) were used as column headers and we generated waaay to many column header separators, namely one for each character in the header line.

shake/compare/Compare.hs

@@ -63,12 +63,12 @@ args =
 
 renderGitLabMarkdown :: (rh -> String) -> (ch -> String) -> (a -> String) -> Table rh ch a -> String
 renderGitLabMarkdown renderRow renderCol renderCell (Table rows cols cells) =
-    unlines $ [header, headerSep] ++ cells'
+    unlines $ [joinCols header, joinCols headerSep] ++ cells'
   where
     prepend x = (x :)
     joinCols xs = "| " ++ intercalate " | " xs ++ " |"
-    header = joinCols $ prepend "" $ map renderRow (headerContents rows)
-    headerSep = joinCols $ map (const " -------- ") header
+    header = prepend "" $ map renderCol (headerContents cols)
+    headerSep = map (const " -------- ") header
     emphasize x = "*"++x++"*"
     cells' = zipWith (\rh row -> joinCols
                                  $ prepend (emphasize $ renderRow rh)