Do not use default branch for new pull requests with source branch only specified

Problem to solve

The link generated when pushing a branch looks as follows: $project/merge_requests/new?merge_request%5Bsource_branch%5D=mybranch

When clicking on the link then GitLab shows the second step of the "New Merge Request" process. GitLab chooses the default branch as target branch by default.

Further details

Usually this is not a big deal, the user can click "Change branches" to change branch if needed and move forward again.

However, for some (large) projects with branches on vastly different states in the history (e.g. a Linux kernel tree with stable 4.4, 4.14 branches) it can be fatal: Is the default branch at 4.4, and I create a patch ontop of 4.14, GitLab tries to calculate the difference between 4.4 and 4.14, which usually leads in a 502.

Proposal

Act conservatively and show the Source/Target branch first. Choose the $default as target branch so the current merge requests behavior is only a click away.

What does success look like, and how can we measure that?

Click on the link after pushing a branch should show the Source/Target branch selection.

Links / references

gitlab-ce!9663