Serialize commit count in diff resource

Review Request #10282 — Created Oct. 26, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
ccf3e36...

Reviewers

We now include the commit_count in the payload for the diff resource
when the DVCS feature is enabled. This allows RBTools to check for
review requests created with history support when doing rbt patch
without making an additional request to either the commit list resource
or the review request resource when unnecessary.

Ran unit tests.