Strip tips out of commit messages from arc backout

Summary:
Fixes T10707. Currently, arc backout creates a commit message which includes questionably-helpful "tips" in the message itself.

Strip these out.

Test Plan:
Used arc backout to revert any commit, then git show to see the generated message.

• Before patch: included tips.
• After patch: no tips.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10707

Differential Revision: https://secure.phabricator.com/D15573