fix git ls-tree parser

File mode is octal not decimal. This broke in the conversion to
attoparsec.

(I've submitted the content of Utility.Attoparsec to the attoparsec
developers.)

Test suite passes 100% now.

Utility/Attoparsec.hs

@@ -0,0 +1,21 @@
+{- attoparsec utility functions
+ -
+ - Copyright 2019 Joey Hess <id@joeyh.name>
+ - Copyright 2007-2015 Bryan O'Sullivan
+ -
+ - License: BSD-3-clause
+ -}
+
+module Utility.Attoparsec where
+
+import qualified Data.Attoparsec.ByteString as A
+import qualified Data.ByteString as B
+
+-- | Parse and decode an unsigned octal number.
+--
+-- This parser does not accept a leading @\"0o\"@ string.
+octal :: Integral a => A.Parser a
+octal = B.foldl' step 0 `fmap` A.takeWhile1 isOctDigit
+  where
+	isOctDigit w = w >= 48 && w <= 55
+	step a w = a * 8 + fromIntegral (w - 48)