Add timing information for commit hooks to push logs

Summary:
Depends on D19779. Ref T13216. The push logs currently record the "hostWait", which is roughly "locking + subprocess cost". We also record locking separately, so we can figure out "subprocess cost" alone by subtracting the lock costs.

However, the subprocess (normally `git receive-pack`) runs hooks, and we don't have an easy way to figure out how much time was spent doing actual `git` stuff vs spent doing commit hook processing. This would have been useful in diagnosing at least one recent issue.

Track at least a rough hook cost and record it in the push logs.

Test Plan: Pushed to a repository, saw a reasonable hook cost appear in the database table.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13216

Differential Revision: https://secure.phabricator.com/D19780

scripts/repository/commit_hook.php

@@ -17,6 +17,8 @@
 // subclasses of PhabricatorConfigSiteSource to read it and build an instance
 // environment.
 
+$hook_start = microtime(true);
+
 if ($argc > 1) {
   $context = $argv[1];
   $context = explode(':', $context, 2);
@@ -35,7 +37,8 @@ if ($argc < 2) {
   throw new Exception(pht('usage: commit-hook <repository>'));
 }
 
-$engine = new DiffusionCommitHookEngine();
+$engine = id(new DiffusionCommitHookEngine())
+  ->setStartTime($hook_start);
 
 $repository = id(new PhabricatorRepositoryQuery())
   ->setViewer(PhabricatorUser::getOmnipotentUser())