Move Differential commit message rendering to dedicated classes

Summary:
Ref T11114. This probably still has some bugs, but survives basic sanity checks.

Continue pulling commit message logic out of CustomField so we can reduce the amount of responsibility/bloat in the classtree and send more code through EditEngine.

Test Plan:

• Called differential.getcommitmessage via API console for various revisions/parameters (edit and create mode, with and without fields, with and without revisions).
• Used --create, --edit and --update modes of arc diff from the CLI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17066