RBTools picks up extra changes with private files in clearcase directories.

When a change happens on a clearcase directory, the contents of the previous
and new versions are retrieved by listing directory content. However, there
could be private files, and these private files should be ignored.

Moreover, this directory comparison mechanism doesn't work in snapshot views.
To do that, we can use `cleartool describe`.

Testing Done:
touch dir/private_files.txt
post-review --revision-range=dir@/main/1:dir@/main/2 --output-diff
dir/private_files.txt is no more listed in diff

Bugs closed: 3178

Reviewed at https://reviews.reviewboard.org/r/5167/