Web IDE does not work on some branch name
### Summary
If a branch is named as following `project_group/project_name#123`, the Web IDE is unable to edit files on that branch.
### Steps to reproduce
- Create a branch named `project_group/project_name#123`
- Edit a file on that branch with Web IDE
### What is the current *bug* behavior?
Gitlab show that banner upon running Web IDE:
### What is the expected *correct* behavior?
Web IDE should be able to edit any file on any branch
#### Results of GitLab environment info
gitlab 12.4.0-ee on-premise
issue