Install pre-commit hooks in Git repositories

Summary:
Ref T4189. T4189 describes most of the intent here:

  - When updating hosted repositories, sync a pre-commit hook into them instead of doing a `git fetch`.
  - The hook calls into Phabricator. The acting Phabricator user is sent via PHABRICATOR_USER in the environment. The active repository is sent via CLI.
  - The hook doesn't do anything useful yet; it just veifies basic parameters, does a little parsing, and exits 0 to allow the commit.

Test Plan:
  - Performed Git pushes and pulls over SSH and HTTP.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4189

Differential Revision: https://secure.phabricator.com/D7682

scripts/repository/commit_hook.php

@@ -0,0 +1,56 @@
+#!/usr/bin/env php
+<?php
+
+$root = dirname(dirname(dirname(__FILE__)));
+require_once $root.'/scripts/__init_script__.php';
+
+$username = getenv('PHABRICATOR_USER');
+if (!$username) {
+  throw new Exception(pht('usage: define PHABRICATOR_USER in environment'));
+}
+
+$user = id(new PhabricatorPeopleQuery())
+  ->setViewer(PhabricatorUser::getOmnipotentUser())
+  ->withUsernames(array($username))
+  ->executeOne();
+if (!$user) {
+  throw new Exception(pht('No such user "%s"!', $username));
+}
+
+if ($argc < 2) {
+  throw new Exception(pht('usage: commit-hook <callsign>'));
+}
+
+$repository = id(new PhabricatorRepositoryQuery())
+  ->setViewer($user)
+  ->withCallsigns(array($argv[1]))
+  ->requireCapabilities(
+    array(
+      // This capability check is redundant, but can't hurt.
+      PhabricatorPolicyCapability::CAN_VIEW,
+      DiffusionCapabilityPush::CAPABILITY,
+    ))
+  ->executeOne();
+
+if (!$repository) {
+  throw new Exception(pht('No such repository "%s"!', $callsign));
+}
+
+if (!$repository->isHosted()) {
+  // This should be redundant too, but double check just in case.
+  throw new Exception(pht('Repository "%s" is not hosted!', $callsign));
+}
+
+$stdin = @file_get_contents('php://stdin');
+if ($stdin === false) {
+  throw new Exception(pht('Failed to read stdin!'));
+}
+
+$engine = id(new DiffusionCommitHookEngine())
+  ->setViewer($user)
+  ->setRepository($repository)
+  ->setStdin($stdin);
+
+$err = $engine->execute();
+
+exit($err);