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 field (this workflow hits both the parse and construct steps).
- Used `arc amend --show` to examine construct output (this does not activate the "edit" mode).
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T3886, T2222
Differential Revision: https://secure.phabricator.com/D8449
