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

src/applications/diffusion/engine/DiffusionCommitHookEngine.php

@@ -0,0 +1,76 @@
+<?php
+
+final class DiffusionCommitHookEngine extends Phobject {
+
+  private $viewer;
+  private $repository;
+  private $stdin;
+
+  public function setStdin($stdin) {
+    $this->stdin = $stdin;
+    return $this;
+  }
+
+  public function getStdin() {
+    return $this->stdin;
+  }
+
+  public function setRepository(PhabricatorRepository $repository) {
+    $this->repository = $repository;
+    return $this;
+  }
+
+  public function getRepository() {
+    return $this->repository;
+  }
+
+  public function setViewer(PhabricatorUser $viewer) {
+    $this->viewer = $viewer;
+    return $this;
+  }
+
+  public function getViewer() {
+    return $this->viewer;
+  }
+
+  public function execute() {
+    $type = $this->getRepository()->getVersionControlSystem();
+    switch ($type) {
+      case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
+        $err = $this->executeGitHook();
+        break;
+      default:
+        throw new Exception(pht('Unsupported repository type "%s"!', $type));
+    }
+
+    return $err;
+  }
+
+  private function executeGitHook() {
+    $updates = $this->parseGitUpdates($this->getStdin());
+
+    // TODO: Do useful things.
+
+    return 0;
+  }
+
+  private function parseGitUpdates($stdin) {
+    $updates = array();
+
+    $lines = phutil_split_lines($stdin, $retain_endings = false);
+    foreach ($lines as $line) {
+      $parts = explode(' ', $line, 3);
+      if (count($parts) != 3) {
+        throw new Exception(pht('Expected "old new ref", got "%s".', $line));
+      }
+      $updates[] = array(
+        'old' => $parts[0],
+        'new' => $parts[1],
+        'ref' => $parts[2],
+      );
+    }
+
+    return $updates;
+  }
+
+}