Implement "Body" field in Herald pre-commit content hooks

Summary: Ref T4195. Adds support for writing rules against commit message bodies.

Test Plan: Pushed git, hg, svn commits and verified their bodies populated correctly in transcripts.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7796