crucible.py pre-commit: Allow posting of specific changelist from Perforce

With Perforce source control system, it is sometimes common for users to have multiple changelists open at one time.

Currently when you use the crucible.py pre-commit script, it will upload all files that are open in all pending changelists.

This feature request is to add the ability to choose a particular changelist to send to Crucible. When choosing a particular change list, only the files in that change list will be uploaded.