Fixes long commit messages overflow viewport in file tree

## What does this MR do?
Fixes long commit messages breaking the table.
It adds back a max-width in `pixels` instead of `%`.

## Are there points in the code the reviewer needs to double check?
No.
## Why was this MR needed?
To fix the overflow of the commit message

## Screenshots (if relevant)
![max_width](/uploads/73af2ffbab29bf6e9bbd9287e9e142a0/max_width.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Fixes #22544

See merge request !6573
Signed-off-by: Rémy Coutable <remy@rymai.me>