avoid git warning on first import of subdir from a remote

git rev-parse --quiet avoids "fatal: Invalid object name" when the
branch does not exist. Git.Ref.tree already returned a Maybe, so callers
already handle those cases themselves.

Git/Ref.hs

@@ -146,7 +146,7 @@ delete oldvalue ref = run
 {- Gets the sha of the tree a ref uses. -}
 tree :: Ref -> Repo -> IO (Maybe Sha)
 tree (Ref ref) = extractSha <$$> pipeReadStrict
-	[ Param "rev-parse", Param ref' ]
+	[ Param "rev-parse", Param "--quiet", Param ref' ]
   where
 	ref' = if ":" `isInfixOf` ref
 		then ref