Fix MR commenting system when new commits pushed

This conmmit fixes wierd behaviour when you see wrong comments about new
commits in merge requests.

Short explanation of a bug:
When you push new commits to project we updated `project.merge_requests`
with comment about push. But `project.merge_requests` includes also
merge requests from forks to origin project. So when you push to master
branch it commented on all merge requests from forks to origin where
source_branch was `master`
Merge branch 'fix-mr-commit-comment' into 'master'

Fix MR commenting system when new commits pushed

Fixes #1608

This conmmit fixes wierd behaviour when you see wrong comments about new
commits in merge requests.

Short explanation of a bug:
When you push new commits to project we updated `project.merge_requests`
with comment about push. But `project.merge_requests` includes also
merge requests from forks to origin project. So when you push to master
branch it commented on all merge requests from forks to origin where
source_branch was `master`

See merge request !1108