Support post-review in Clearcase snapshot view

Currrently run RBTools in snapshot view will get error message saying "To
generate diff using parent branch or by passing revision ranges, you must use a
dynamic view.

So I made some change to support snapshot view post-review, as it will be useful
for clearcase users working on snapshot views. The idea is to use
	cleartool get -to dest-pname pname
to copy the revision to temp dir and then run diff on temp old new files.

Testing done:
Tested on Clearcase 7.1.2 snapshot view, the diff could be uploaded just like
dynamic view.

Reviewed at http://reviews.reviewboard.org/r/3822/