T.P.App.Opt: fix logic bug in fullDefaultsPath.

Previously we would (also) search the default user data directory
for a defaults file, even if a different user data directory
was specified using `--data-dir`.  This was a mistake; if
`--data-dir` is used, the default user data directory should
not be searched.

src/Text/Pandoc/App/Opt.hs

@@ -707,9 +707,10 @@ fullDefaultsPath dataDir file = do
   let fp = if null (takeExtension file)
               then addExtension file "yaml"
               else file
-  defaultDataDir <- liftIO defaultUserDataDir
-  let defaultFp = fromMaybe defaultDataDir dataDir </> "defaults" </> fp
-  fromMaybe fp <$> findM fileExists [fp, defaultFp]
+  let searchpath = fp : case dataDir of
+                          Nothing -> []
+                          Just d  -> [d </> "defaults" </> fp]
+  fromMaybe fp <$> findM fileExists searchpath
 
 -- | In a list of lists, append another list in front of every list which
 -- starts with specific element.