Do not update updated_at on an issue when it is mentioned elsewhere

Summary

An issue's updated_at timestamp is updated when it is mentioned elsewhere. This is potentially the cause of some customers being concerned and reporting that old or closed issues have been mysteriously updated even though there has been no apparent activity.

Steps to reproduce

1. Create an issue
2. Wait for some time, then create another issue
3. In the new issue, post a comment mentioning the first issue
4. The first issue's updated_at timestamp is updated

What is the current bug behavior?

An issue's updated_at timestamp gets updated when it is mentioned elsewhere.

What is the expected correct behavior?

The issue's updated_at timestamp should not get updated when it is mentioned elsewhere as the issue was not actually updated in any form.

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:env:info)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)