Wrong commit back-ref's timestamp in issues upon push
### Summary
Commit mention should be inserted at the timestamp the commit has occured - not the push.
See http://gitlabtest.ow2.org/clif/test2/issues/314863
### Steps to reproduce
- Update the variable PROCESS_COMMIT_LIMIT to 10000 in [git_push_service.rb](/gitlab-org/gitlab-ce/master/opt/gitlab/embedded/service/gitlab-rails/app/services/git_push_service.rb) and restart GitLab
- Create project
- Import existing issues (here it comes from JIRA)
- Initialize the repository with a first push
- Push an existing repository
### Example Project
See http://gitlabtest.ow2.org/clif/test2
### What is the current *bug* behavior?
Commit mention timestamp match the push event timestamp.
### What is the expected *correct* behavior?
Commit mention timestamp to match the *commit* timestamp.
### Relevant logs and/or screenshots
See http://gitlabtest.ow2.org/clif/test2
#### Results of GitLab environment info
<details>
<summary>Expand for output related to GitLab environment info</summary>
<pre>
System information
System: Debian 8.9
Current User: git
Using RVM: no
Ruby Version: 2.3.5p376
Gem Version: 2.6.13
Bundler Version:1.13.7
Rake Version: 12.1.0
Redis Version: 3.2.5
Git Version: 2.13.6
Sidekiq Version:5.0.4
Go Version: unknown
GitLab information
Version: 10.1.3
Revision: e6142bc
Directory: /opt/gitlab/embedded/service/gitlab-rails
DB Adapter: postgresql
URL: http://gitlabtest.ow2.org
HTTP Clone URL: http://gitlabtest.ow2.org/some-group/some-project.git
SSH Clone URL: git@gitlabtest.ow2.org:some-group/some-project.git
Using LDAP: no
Using Omniauth: no
GitLab Shell
Version: 5.9.3
Repository storage paths:
- default: /var/opt/gitlab/git-data/repositories
Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks
Git: /opt/gitlab/embedded/bin/git
</pre>
</details>
issue