fix failing quickcheck properties

QuickCheck 2.10 found a counterexample eg "\929184" broke the property.

As far as I can tell, Git.Filename is matching how git handles encoding
of strange high unicode characters in filenames for display. Git does
not display high unicode characters, and instead displays the C-style
escaped form of each byte. This is ambiguous, but since git is not
unicode aware, it doesn't need to roundtrip parse it.

So, making Git.FileName's roundtrip test only chars < 256 seems fine.

Utility.Format.format uses encode_c, in order to mimic git, so that's
ok.

Utility.Format.gen uses decode_c, but only so that stuff like "\n"
in the format string is handled. If the format string contains C-style
octal escapes, they will be converted to ascii characters, and not
combined into unicode characters, but that should not be a problem.
If the user wants unicode characters, they can include them in the
format string, without escaping them.

Finally, decode_c is used by Utility.Gpg.secretKeys, because gpg
--with-colons hex-escapes some characters in particular ':' and '\\'.
gpg passes unicode through, so this use of decode_c is not a problem.

This commit was sponsored by Henrik Riomar on Patreon.

Git/Filename.hs

@@ -8,9 +8,10 @@
 
 module Git.Filename where
 
+import Common
 import Utility.Format (decode_c, encode_c)
 
-import Common
+import Data.Char
 
 decode :: String -> FilePath
 decode [] = []
@@ -23,6 +24,11 @@ decode f@(c:s)
 encode :: FilePath -> String
 encode s = "\"" ++ encode_c s ++ "\""
 
-{- for quickcheck -}
-prop_isomorphic_deencode :: String -> Bool
-prop_isomorphic_deencode s = s == decode (encode s)
+{- For quickcheck. 
+ -
+ - See comment on Utility.Format.prop_encode_c_decode_c_roundtrip for
+ - why this only tests chars < 256 -}
+prop_encode_decode_roundtrip :: String -> Bool
+prop_encode_decode_roundtrip s = s' == decode (encode s')
+  where
+	s' = filter (\c -> ord c < 256) s