Add support for ClearCase lbtype comparison

Identify all elements belonging to previous and current labels thanks to
cleartool find -exec cleartool desc command. Then for each element, attach
these versions or default one "/main/0" (when a file was added or removed). Add
only versions not matching to the changeset. When one label is provided, set a
temporary label on current directory before proceeding to label comparison.
This label is removed when a error happen eg. current directory contains
checked out elements.

Testing Done:
- Tested done within a clearcase dynamic view and a snapshot view from a linux
  machine

- rbt diff generation between one label and current => OK
- rbt diff generation between two labels => OK

- rbt post on a reviewboard server 2.0.0 => OK
- rbt post on a reviewboard server 1.7.6 with this patch using rbtools
  release-0.6.2 + 5defd0c + 30b0194

- error message when root dir is checked out => OK
- error message when any element under root dir is checked out => OK

  Label comparison failed because:
  ClearCase backend cannot set label when some elements are checked out:
  <element xxx>

Bugs closed: 3179

Reviewed at https://reviews.reviewboard.org/r/6500/
Merge branch 'release-0.6.x'