Add "is merge commit" Herald field for pre-commit rules

Summary:
Ref T4195. This allows you to write rules which disallow merge commits.

Also make the reject message a little more useful.

Test Plan:

remote: This push was rejected by Herald push rule H27.
remote: Change: commit/daed0d448404
remote:   Rule: No Merges
remote: Reason: No merge commits allowed. If you must push a merge, include "@force-merge" in the commit message.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7809