Setting merge commit with semi-linear history method rebases even if that branch contains other merges

Summary

(Summarize the bug encountered concisely)

Steps to reproduce

• Have a repo with the default branch master.
• Add some commits.
• Create a fix-1 branch.
• Add some commits.
• Create a fix-2 branch from fix-1.
• Add some commits.
• Merge the fix-2 branch into fix-1.
• Create a request for merging fix-1 into master.
• Add some commits to master.

Example Project

(If possible, please create an example project here on GitLab.com that exhibits the problematic behavior, and link to it here in the bug report)

(If you are using an older version of GitLab, this will also determine whether the bug is fixed in a more recent version)

What is the current bug behavior?

A rebase button shows up in the MR.

What is the expected correct behavior?

As far as I know, the rebase won't work when the branch contains merges.

Relevant logs and/or screenshots

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Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

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(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

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(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)