Fix project overview bugs when default branch collides with tag

Fix problems around commit messages and last commits:
- upcase ref_type for graphql requests
- make ref_type case insensitive for api requests
- make project overview implicitly request a branch ref_type

Fix problems around buttons for adding files:
- make head_tree always refer to a branch (the default branch)
  rather than situationally to a tag

Contributes to: https://gitlab.com/gitlab-org/gitlab/-/issues/419973
Changelog: fixed