bash_completion: support GNU sed 3 and non-GNU sed
replaced command 'Q' introduced in GNU sed 4, in order to support
platforms with older sed.
Is there an error message when trying to diff non-existing files?
not: No such file or directory
found: No such file or directory
Is there an error message when trying to add non-existing files?
not: No such file or directory
found: No such file or directory