Never try to run README as a commit hook

Summary:
Fixes T4960. Users `chmod +x` this, and then bash chokes on it.

Phabricator "owns" this file anyway, so there is no real ambiguity here: this should never be a hook script.

Test Plan:
  - Did `chmod +x README`.
  - Made a commit.
  - Added `z.sh`, got blocked.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4960

Differential Revision: https://secure.phabricator.com/D8981