Git post commit hook payloads has empty commits field for large commits (BB-8636)

We use the post commit hook to read details of the last commit made. However, for commits with large number of files, the payload we receive has an empty "commits" list

Here's the repro:

1. Clone the repository at https://bitbucket.org/kudutest/bitbucketlargepayload. It has a single commit that includes about 5000 files.
2. Create a new Git repository on Bitbucket
3. In this new repo, set up a POST hook from the services tab to point to a receiver (I used http://requestb.in/)
4. Push the cloned repository to the new repo

Expected: The payload contains specifics about the commit

Actual: The commits field is empty. Here's what the payload looks like:

{ "canon_url": "https://bitbucket.org", "commits": [], "repository":

{ "absolute_url": "/kudutest/bitbucketlargepayload/", "fork": false, "is_private": false, "name": "BitbucketLargePayload", "owner": "kudutest", "scm": "git", "slug": "bitbucketlargepayload", "website": "" }

, "truncated": true, "user": "kudutest" }

Consequent commits with fewer files correctly populate the "commits" field. I'm not sure if this is a recent change in behavior or if it has always been the case.