A branch is considered already pushed if its HEAD commit hash matches
the commit hash of one of the remote tracking branches:
`refs/remotes/<remote>/<branch>`
This mechanism allows the user to:
- Choose a remote as push target for their branch different than the one
that would automatically get chosen for them by `pr create`;
- Avoid a `git push` operation (and its associated git hooks) if a push
was not necessary in the first place.
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