gh stack rebase doubles remote prefix for remote-qualified trunk

[windsornguyen]

Reproduction

With origin/main present:

$ gh stack init --base origin/main layer-a layer-b
✓ Adopted 2 branches: origin/main <- layer-a <- layer-b

$ gh stack rebase --remote origin
✓ Fetched origin
✗ could not create local trunk branch origin/main from origin/origin/main: failed to run git: fatal: not a valid object name: "origin/origin/main"

Expected

init and rebase agree on the accepted trunk name.

Environment

gh stack 0.0.7