'Open in GitHub' doesn't work in diff view

[jankeromnes]

Describe the bug
Using 'Open in GitHub' from the diff view takes you to a broken GitHub URL with the right lines but no file name.

To Reproduce

1. Open a pull request in Gitpod, e.g. https://gitpod.io/#https://github.com/theia-ide/theia/pull/4166
2. Select code from somewhere in the diff view
3. Right-click on the selection, and pick 'Open in GitHub'

This takes you to a broken GitHub URL with the right lines but no file name, e.g.:

https://github.com/jankeromnes/theia/tree/12eeb92bf750da5a7bfe9d9cdb6c7e6b64dc0251#L181-L183

Expected behavior

The GitHub URL should also have the file name, e.g.:

https://github.com/jankeromnes/theia/blob/12eeb92bf750da5a7bfe9d9cdb6c7e6b64dc0251/packages/navigator/src/browser/navigator-contribution.ts#L181-L183

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.