Pull Request defaults to `upstream` instead of `origin` as base branch.

[mikebronner]

Type: Bug

1. Set up a repo that has both an origin and upstream reference.
2. Create new branch from another branch.
3. Make some changes and commit them.
4. Create a PR using the extension.
5. Note that the PR defaults to the upstream repo, instead of the origin repo.

I expect the PR to default to the origin repo, to avoid accidental PRs to the upstream repo (which unfortunately is happening all the time because of this bug).

Extension version: 0.111.2025060504
VS Code version: Code 1.100.3 (258e40fedc6cb8edf399a463ce3a9d32e7e1f6f3, 2025-06-02T13:30:54.273Z)
OS version: Darwin arm64 24.5.0
Modes:

System Info

Item	Value
CPUs	Apple M1 Ultra (20 x 2400)
GPU Status	2d_canvas: enabled canvas_oop_rasterization: enabled_on direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: enabled raw_draw: disabled_off_ok skia_graphite: disabled_off video_decode: enabled video_encode: enabled webgl: enabled webgl2: enabled webgpu: enabled webnn: disabled_off
Load (avg)	2, 2, 2
Memory (System)	64.00GB (1.34GB free)
Process Argv	--crash-reporter-id 1a294368-d2ff-4366-8373-082c137ba924
Screen Reader	no
VM	0%

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