PR diff does not always recognize PR

[akosyakov]

See There is no pull request.:

It's reproducible for: eclipse-theia/theia#5326

[akosyakov]

Also files cannot be opened from the diff view.

As a workaround one can reopen it from the PR view then it works.

[akosyakov]

Probably the important detail is that i've rebased the PR for review after starting the workspace.

[stale]

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