xzgrep: exit 0 when at least one file matches.

Mimic the original grep behavior and return exit_success when
at least one xz compressed file matches given pattern.

Original bugreport:
https://bugzilla.redhat.com/show_bug.cgi?id=1108085

Thanks to Pavel Raiskup for the patch.
xzgrep: Add a test for the previous fix.

This is a simplified version of Pavel Raiskup's
original patch.