respect maintainer_can_modify settings on PRs

[akosyakov]

I can push as a collaborator to PR:

Gitpod does not acknowledge it:

I still can push from a terminal though and via Git UI.

[AlexTugarev]

I ran some tests on the GH API, and the only hint to determine this is checking the result of GET /repos/:owner/:repo/pulls/:number for maintainer_can_modify: boolean.

We need to improve the validation of permissions in case of a PR context (origin != upstream):

• accept if maintainer_can_modify is true
• notify otherwise

[akosyakov]

I again stumbled over it in eclipse-theia/theia#7610

git push -f is good:

But UI tells me oppositive:

[AlexTugarev]

Revision of the git credential helper to support this is in progress. The happy path was working, but it needs to be tested and fine tuned. One unknown thing remaining ist, that git tends to cache results, which shouldn't be as it would lead to false results whenever you adjust and retry.

[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.