Merge Request creation times out for certain branches

Summary

If one user tries to create a MR against another branch and the change list is big enough, the MR form will never load and a 502 error will be shown.

Steps to reproduce

This is noticeable in two different UI views:

• Using the blue "Create merge request" you usually have on top of the project whenever you push a change to a fork, as this tries to show the form directly with the affected branch with a humongous amount of changes than cannot be loaded on a reasonable amount of time.
• Using the usual green "New merge request" button and selecting then the affected branch.

We cannot provide how many changes or how big they need to be in order to get this behaviour, but usually it happens with +999 commits behind and ahead.

What is the current bug behavior?

Users get:

502 Whoops, GitLab is taking too much time to respond.

What is the expected correct behavior?

Gitlab should either limit the amount of changes to be shown or add some time limited mechanism to avoid this error.