Bug on solving conflict with the UI

Summary

Solving merge conflict with the UI merge destination into source.

Steps to reproduce

  1. Create a conflict on a file on branches A and B
  2. Modify a different file on branch B
  3. Create merge request A -> B
  4. Solve the conflict using the UI
  5. The UI generated conflict commit will merge the other modified file on B into A

What is the current bug behavior?

After fixing a conflict via the UI, gitlab incorrectly merges the destination branch into the source branch (before performing the intended merge of the source into the destination).

What is the expected correct behavior?

Not merging destination into source

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com /label reproduced 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