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