Add commit message to JIRA mention comment

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/13677

If a JIRA issue is mentioned in a commit, add the commit message to the JIRA comment from GitLab. This could look something like:

USER mentioned this issue in abcd123:
“Git commit message”

Moving here from gitlab-org/gitlab-ee#50 and gitlab-org/gitlab-ee#124