R1:ae3c1f781939

Perform commit message parsing and construction with new CustomFields

Summary: Ref T2222. Ref T3886. Converts parsing and construction of commit messages to be driven by CustomField.

Test Plan:
This is a huge, messy change. I've made an effort to test it exhasutively, but suspect I probably missed a few behaviors. Roughly:

- Enumerted all current fields (fields implementing `shouldAppearOnCommitMessage()`) and tried to test them one by one.
- Used `arc diff --edit` repeatedly to manipulate each…