bugzilla 1754: h5diff: support comparing through links.
(original check-in svn revision #18164)
Description:
fix the hang issue in parallel mode when compare external-link.
add --no-dangling-links option.
add test cases (#450-#459) relate to the new option.
improve test script to check exit code.
update --help relate to the new options.
correct some indentations.
Tested:
h5committest (jam, amani and linew)
