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