branch creation message in hipchat notification inaccurate

This is probably related to STASH-6910, but not marking as a duplicate as I'm not sure.

If you create a new empty branch and push to stash, the hipchat notification is:

5+ commits pushed to branch ...
- 1 desc
- 2 desc
- 3 desc
- 4 desc
- 5 desc
See more

The code in com.atlassian.stash.internal.hipchat.hook.HipChatNotificationRenderer#getCommits doesn't work properly for new branches, as it gets all commits from 00000 to branch head.

Tested on 3.7 but the code is still unchanged in 3.8.0, which is latest source I have access to atm.