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

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).