Fix --file option (#1004)

The `--file` option was essentially broken since it passed in the
file path instead of the directory, meaning that transitive imports
would not be computed correctly (they'd be off by one path component).

dhall/src/Dhall/Main.hs

@@ -70,6 +70,7 @@ import qualified Options.Applicative
 import qualified Paths_dhall as Meta
 import qualified System.Console.ANSI
 import qualified System.IO
+import qualified System.FilePath
 import qualified Text.Dot
 import qualified Data.Map
 
@@ -299,7 +300,7 @@ command (Options {..}) = do
           where
             file = case maybeFile of
                 Just "-" -> "."
-                Just f   -> f
+                Just f   -> System.FilePath.takeDirectory f
                 Nothing  -> "."
 
     let handle =