OsPath build flag no longer depends on filepath-bytestring

However, filepath-bytestring is still in Setup-Depends.
That's because Utility.OsPath uses it when not built with OsPath.
It would be maybe possible to make Utility.OsPath fall back to using
filepath, and eliminate that dependency too, but it would mean either
wrapping all of System.FilePath's functions, or using `type OsPath = FilePath`

Annex.Import uses ifdefs to avoid converting back to FilePath when not
on windows. On windows it's a bit slower due to that conversion.
Utility.Path.Windows.convertToWindowsNativeNamespace got a bit
slower too, but not really worth optimising I think.

Note that importing Utility.FileSystemEncoding at the same time as
System.Posix.ByteString will result in conflicting definitions for
RawFilePath. filepath-bytestring avoids that by importing RawFilePath
from System.Posix.ByteString, but that's not possible in
Utility.FileSystemEncoding, since Setup-Depends does not include unix.
This turned out not to affect any code in git-annex though.

Sponsored-by: Leon Schuermann

Utility/FileSystemEncoding.hs

@@ -23,7 +23,6 @@ module Utility.FileSystemEncoding (
 
 import qualified GHC.IO.Encoding as Encoding
 import System.IO
-import System.FilePath.ByteString (RawFilePath, encodeFilePath, decodeFilePath)
 import qualified Data.ByteString as S
 import qualified Data.ByteString.Lazy as L
 #ifdef mingw32_HOST_OS
@@ -37,6 +36,9 @@ import Data.Char
 import Data.List
 #endif
 
+-- | A literal file path
+type RawFilePath = S.ByteString
+
 {- Makes all subsequent Handles that are opened, as well as stdio Handles,
  - use the filesystem encoding, instead of the encoding of the current
  - locale.
@@ -90,9 +92,7 @@ encodeBL = L8.fromString
 decodeBS :: S.ByteString -> FilePath
 #ifndef mingw32_HOST_OS
 -- This does the same thing as System.FilePath.ByteString.decodeFilePath,
--- with an identical implementation. However, older versions of that library
--- truncated at NUL, which this must not do, because it may end up used on
--- something other than a unix filepath.
+-- with an identical implementation.
 {-# NOINLINE decodeBS #-}
 decodeBS b = unsafePerformIO $ do
 	enc <- Encoding.getFileSystemEncoding
@@ -104,9 +104,7 @@ decodeBS = S8.toString
 encodeBS :: FilePath -> S.ByteString
 #ifndef mingw32_HOST_OS
 -- This does the same thing as System.FilePath.ByteString.encodeFilePath,
--- with an identical implementation. However, older versions of that library
--- truncated at NUL, which this must not do, because it may end up used on
--- something other than a unix filepath.
+-- with an identical implementation.
 {-# NOINLINE encodeBS #-}
 encodeBS f = unsafePerformIO $ do
 	enc <- Encoding.getFileSystemEncoding
@@ -116,10 +114,10 @@ encodeBS = S8.fromString
 #endif
 
 fromRawFilePath :: RawFilePath -> FilePath
-fromRawFilePath = decodeFilePath
+fromRawFilePath = decodeBS
 
 toRawFilePath :: FilePath -> RawFilePath
-toRawFilePath = encodeFilePath
+toRawFilePath = encodeBS
 
 {- Truncates a FilePath to the given number of bytes (or less),
  - as represented on disk.