Blame hover / PR-defaults polling fails with "is not a GitHub repo" when branch upstream is a local sibling branch (stacked-branch workflows)

[dave-rudolf-aurora]

Describe the bug

When a branch's upstream is configured to track a local sibling branch rather than a remote-tracking branch — the pattern used by stacked-PR tooling (e.g. Graphite, git-branchless, or similar internal stacking tools), where branch.<name>.remote is set to . and branch.<name>.merge points at refs/heads/<other-local-branch> — the extension's background polling repeatedly throws:

Error: The upstream ref <ref> for branch <branch> is not a GitHub repo.
    at gy.getOrigin
    at async gy.getPullRequestDefaults
    at async by.getPullRequestsForQuery
    at async R6._pollForChanges

As a side effect, the native VS Code git blame hover (git.blame.editorDecoration.enabled) stops showing its full-commit-message hover popup on any file while that branch is checked out — only the one-line inline decoration still renders. Switching to a branch whose upstream is a normal origin remote branch immediately restores the full hover popup, isolating the cause to this upstream configuration rather than anything else.

To Reproduce

1. In a repo with a GitHub remote origin, create two local branches, e.g. feature-a and feature-b (feature-b branched from feature-a).
2. Set feature-b's upstream to track feature-a locally instead of a remote:

   git config branch.feature-b.remote .
   git config branch.feature-b.merge refs/heads/feature-a
   

3. Open the repo in VS Code with this extension installed and signed in, with feature-b checked out.
4. Open View → Output → Log (Extension Host) and observe the repeated error above.
5. Enable git.blame.editorDecoration.enabled and hover over a blame-decorated line — no full-commit hover panel appears (only the inline one-liner).

Expected behavior

The extension should recognize that the branch's upstream isn't itself a hosted remote, skip/no-op the PR-defaults resolution for that branch gracefully, and not interfere with the native git blame hover on other branches or files.

Environment

• VS Code 1.133.0
• GitHub Pull Requests and Issues extension 0.162.0
• Ubuntu 22.04.5 LTS, Linux kernel 6.8.0-136-generic, x86_64
• git 2.52.0