Allow push to hidden refs at pull/*/head

[JojOatXGME]

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I recently learned that you can checkout pull requests locally without adding the fork as another remote.

> git fetch origin pull/42/head:pull/42
> git checkout pull/42

or

> git config --local --add remote.origin.fetch '+refs/pull/*/head:refs/remotes/origin/pull/*'
> git fetch
> git checkout pull/42

However, according to the documentation, these refs are read-only. Assuming the PR is still open and the creator of the PR allows changes to the pull request branch, it would be nice if I could push to these references.