tests: make pdiff return appropriate exit code at comparison of files Before this patch, pdiff script returns 0, even if diff is detected. This issue doesn't cause failure of tests using it, if it is invoked via extdiff extension, because extdiff itself examines changes between specified revisions and decides exit code. BTW, this patch ignores recursive comparison case, because: - there is no portable way for current while-read based implementation to return 1 at detecting changes - it isn't yet needed to replace direct "diff -r" invocation by pdiff for portability