portability fixes for some tests
Apparently the ls on OS X 10.3.9 exits with success even if the file
given on the command line doesn't exist
Is there an error message when trying to diff non-existing files?
found: No such file or directory
not: No such file or directory
Is there an error message when trying to add non-existing files?
found: No such file or directory
not: No such file or directory