Allow editing of files on commit ref
### Problem to solve
<!--- What problem do we solve? -->
Editing files when on a commit ref is currently not possible and introduces unneeded friction when using the GitLab web ide or normal editing capabilities.

### Target audience
<!--- For whom are we doing this? Include either a persona from https://design.gitlab.com/getting-started/personas or define a specific company role. e.a. "Release Manager" or "Security Analyst". Use the persona labels as well https://gitlab.com/groups/gitlab-org/-/labels?utf8=%E2%9C%93&subscribed=&search=persona%3A -->
- gitlab-ce~9335216
### Further details
<!--- Include use cases, benefits, and/or goals (contributes to our vision?) -->
### Proposal
<!--- How are we going to solve the problem? Try to include the user journey! -->
Allow editing, a new branch could be created from that ref
### What does success look like, and how can we measure that?
<!--- Define both the success metrics and acceptance criteria. Note thet success metrics indicate the desired business outcomes, while acceptance criteria indicate when the solution is working correctly. If there is no way to measure success, link to an issue that will implement a way to measure this -->
### Links / references
issue