When a perforce changeset is submitted, it will often be given a new number in

order to ensure that changeset IDs are monotonically increasing. One

long-standing request we've had is to update the 'change' field on review

requests when this happens.
Without a hook on the SCM side, we don't have any notification that a changeset

was submitted, but it turns out that we do already query the perforce server to

determine whether or not to display "(pending)" next to the change number. This

change adds some logic to find whether or not the change number was updated,

and if so, store the new value. As part of this, we also now cache the

information on whether a changeset is pending or not.
I've also removed the now-unused get_pending_changesets() method.

Created and submitted several perforce changesets, and saw that the 'commit_id'

field on both the review request and any drafts was updated properly when the

page was reloaded after submitting the change. Verified that caching is working

as expected.
Ran unit tests.