Do not start a discussion on commit comments if the source is a system note

Slack (GitLab internal): https://gitlab.slack.com/archives/C02PF508L/p1537457020000100

Currently, we start a discussion on an MR page if one of the commits that is part of that MR is mentioned somewhere. This is a faux discussion to hold all the commit comments, and are mirrored in the commit page also. However, we may want to skip this if the cause is a system note. That is, I mentioned a commit associated to an MR in a comment on the same MR - I don't want a discussion to be started because of that.

/cc @DouweM