Apply "enormous changes" rules to pre-commit content rules too

Summary:
Fixes T4276. This adds "Change is enormous" to pre-commit content rules so we can, e.g., just reject these and not worry about them elsewhere.

Also, use the same numeric limits across the mechanisms so there's a consistent definition of an "enormous" changeset.

Test Plan:
  - Set enormous limit to 15 bytes, pushed some changes, got blocked by a rule.
  - Set it back, pushed OK.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4276

Differential Revision: https://secure.phabricator.com/D7887