Allow GitLab to pass a commit hash to JIRA, if a ticket is mentioned

ZenDesk: https://gitlab.zendesk.com/agent/tickets/9532

One of our customers would really like if GitLab could send a bit more information about what is happening when it talks to JIRA.

He would like for at least the commit ID to be passed to JIRA, if a ticket is mentioned in a commit, instead of GitLab just posting that something happened.

What do you guys think?

/cc @JobV @rspeicher @marin