R1:24926f9453e5

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…