Reports status to wrong commit when receiving two pushes within short timeframe

### Context
- **Gitlab plugin version**: 1.4.5
- **Gitlab version**: 9.0.2-ee
- **Jenkins version**: 1.609.3
- **Job type**: Freestyle

### Logs & Traces

Please see https://gist.github.com/meeee/4f939434812c75d42ba127b6bed99b82

### Problem description

I'm using the Gitlab API to automatically create a branch and make a commit on the branch, which both happens within a few seconds (< 3 seconds).

As visible in the logs, Jenkins receives a hook for branch creation (commit A, 5bb2a383) and a hook for the push on the branch (commit B, 084ae98c), 5 seconds apart. It then continues to run the job for commit B, but posts statuses for commit A. Here's a summary of the logs, written from the perspective of Jenkins:

* Receives push for branch creation (commit A)
* Creates status for commit A (pending)
* Receives push for commit adding .gitlab-ci.yml (commit B)
* Creates status for commit B (pending)
* Runs Jenkins job with commit B, but thinks it's running commit A
* Loads Commit information for commit A
* Creates status for commit A (running)
* Loads commit information for commit A
* Creates status for Commit A (success), although Jenkins tested commit B
* Commit B stays in status 'pending' forever

I would have expected Jenkins to both test commit A and commit B after another and push the statuses for each build to correct commit in Gitlab.

Waiting 10 seconds between creating the branch and creating the commit works around the problem.