Fix front-end for branches that happen to contain urlencoding escape characters (e.g. %)

_Originally opened at !3574 by @ewiltshi._

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Adding a branch with a name like "foo%20bar" (via command-line) and setting it to the project's default branch causes the project "show" page to 404 for that project. "assign_ref_vars" unescapes the branch name ("foo%20bar" ==> "foo bar"), making GitLab look for a non-existent branch.
This MR adds logic to skip the URL unescaping step in "assign_ref_vars".

Fixes #10772, #14992, #15304.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [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)

See merge request !5770