use openTempFile from file-io

And follow-on changes.

Note that relatedTemplate was changed to operate on a RawFilePath, and
so when it counts the length, it is now the number of bytes, not the
number of code points. This will just make it truncate shorter strings
in some cases, the truncation is still unicode aware.

When not building with the OsPath flag, toOsPath . fromRawFilePath and
fromRawFilePath . fromOsPath do extra conversions back and forth between
String and ByteString. That overhead could be avoided, but that's the
non-optimised build mode, so didn't bother.

Sponsored-by: unqueued

Command/Export.hs

@@ -312,12 +312,12 @@ performExport r srcrs db ek af contentsha loc allfilledvar = do
 	sent <- tryNonAsync $ if not (isGitShaKey ek)
 		then tryrenameannexobject $ sendannexobject
 		-- Sending a non-annexed file.
-		else withTmpFile "export" $ \tmp h -> do
+		else withTmpFile (toOsPath "export") $ \tmp h -> do
 			b <- catObject contentsha
 			liftIO $ L.hPut h b
 			liftIO $ hClose h
 			Remote.action $
-				storer tmp ek loc nullMeterUpdate
+				storer (fromRawFilePath (fromOsPath tmp)) ek loc nullMeterUpdate
 	let failedsend = liftIO $ modifyMVar_ allfilledvar (pure . const (AllFilled False))
 	case sent of
 		Right True -> next $ cleanupExport r db ek loc True