Fix exit status of "xzdiff foo.xz bar.xz".

Fix exit status of "xzdiff foo.xz bar.xz".

xzdiff was clobbering the exit status from diff in a case
statement used to analyze the exit statuses from "xz" when
its operands were two compressed files. Save and restore
diff's exit status to fix this.

The bug is inherited from zdiff in GNU gzip and was fixed
there on 2009-10-09.

Thanks to Jonathan Nieder for the patch and
to Peter Pallinger for reporting the bug.