Highlight file when opening a merge request in the Web IDE

Problem to solve

When opening a merge request in the Web IDE we open the changed files associated with the merge request. While this could be multiple files, we do default to displaying one of the files. The file defaulted to open is not shown as highlighted in the file tree. This is inconsistent with the normal flow where the active file is highlighted in the tree as well. This forces users to select another tab and then reselect the original tab to highlight the file.

Proposal

Highlight the file defaulted as open when opening merge requests.

Investigation

As mentioned in this comment, it looks like this "Problem to solve" ☝ is actually a symptom connected with even more serious issues.