compute: add response to OUTPUT

This allows rejecting output filenames that are outside the repository,
and also handles converting eg "-foo" to "./-foo" to prevent a command
that it's passed to interpreting the output filename as a dashed option.

Utility/SafeCommand.hs

@@ -10,6 +10,7 @@
 module Utility.SafeCommand (
 	CommandParam(..),
 	toCommand,
+	toCommand',
 	boolSystem,
 	boolSystem',
 	boolSystemEnv,