Can not open file context sometimes for specific filenames in GitLab

Bug description

Although we've removed the file context from the docs, opening a file directly from the tree in Gitpod works as expected.

However, this does not work sometimes for specific filenames in GitLab.

See relevant issue in https://gitlab.com/gitlab-org/gitlab/-/issues/354263 and relevant discussion (internal).

Steps to reproduce

For example, try the two following example URLs:

1. https://gitpod.io/#https://gitlab.com/gitpod-io/gitlab/-/tree/master/.gitpod.yml
2. https://gitpod.io/#https://gitlab.com/gitpod-io/gitlab/-/tree/master/yarn.lock

Both fail with the following error on the workspace start page:

GitLab ApiError on searching for possible branches for gitpod-io/gitlab/tree/master/.gitpod.yml: GitLabApiError: GitLab Request Error: The branch refname is invalid

Workspace affected

No response

Expected behavior

No response

Example repository

No response

Anything else?

No response

Start by reproducing the two GitLab URLs in the issue and trace the workspace start-page handling of the GitLab branch-ref error. The fix is complete when both the .gitpod.yml and yarn.lock URLs open successfully without the invalid branch refname error.