hdf5

[svn-r24480] Description:
    Bring r24479 from 1.8 branch to trunk:

    Fix ph5diff worker command handling so it can correctly respond to
situations where there's no work to perform.  Also, a few minor code cleanups.

Tested on:
    Mac OSX/64 10.9.0 (amazon) w/parallel
    (too minor to require h5committest)