Remove branch name from pr namespace.

[ejoffe]

Pull requests are only matched on commit-id. This should allow easy sharing of stacks between multiple users.

fixes #308

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Stack:

• 314

• 311

• 313 ⬅

• 310

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.

[ejoffe]

✓ Commit merged in pull request #314