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