Ignore empty lines in --exclude-rx-from files

Thanks to Daniel Hahler Daniel Hahler <git@thequod.de> for suggesting
the change.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>
parse_excludes: drop empty --exclude-from paths

Empty paths shouldn't match during drecurse, but there's no point in
including them.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>