R1:ae7488f71005

Drive commit message rendering from field specifications

Summary:
When rendering commit messages, drive all the logic through field specification
classes instead of the hard-coded DifferentialCommitMessageData class. This
removes DifferentialCommitMessageData and support classes.

Note that this effectively reverts D546, and will cause a minor break for
Facebook (Task IDs will no longer render in commit messages generated by "arc
amend", and will not be editable via "arc diff --edit"). This can be…