test: stabilize TestRebase_interactiveRetryPreservesTerminal on Windows

[ed-irl]

Summary

• TestRebase_interactiveRetryPreservesTerminal flakes on Windows because the test relies on a goroutine sleeping 20ms before removing a fake .git/index.lock. On Windows runners the scheduler doesn't always release the goroutine before the retry path runs — when it doesn't, the test sees the lock still present and fails.
• Replace the goroutine + sleep with atomic lock removal in the first mock call. The lock is removed before the mock returns the lock-conflict error, so when the retry path executes there is no timing dependency to lose.

Before

go func() {
    time.Sleep(20 * time.Millisecond)
    _ = os.Remove(lockPath)
}()

mockExec.EXPECT().
    Run(gomock.Any()).
    DoAndReturn(func(cmd *exec.Cmd) error {
        _, _ = fmt.Fprintln(cmd.Stderr, "fatal: Unable to create '.git/index.lock'")
        return &exec.ExitError{}
    })

After

mockExec.EXPECT().
    Run(gomock.Any()).
    DoAndReturn(func(cmd *exec.Cmd) error {
        // Remove the lock before returning the lock-conflict
        // error so the retry path observes a clean state.
        _ = os.Remove(lockPath)
        _, _ = fmt.Fprintln(cmd.Stderr, "fatal: Unable to create '.git/index.lock'")
        return &exec.ExitError{}
    })

No sleep, no goroutine, no race.

Why this matters

The flake masked CI on multiple unrelated PRs in the current stack (e.g. #1221), wasting workflow runs and obscuring real failures. Fix is independent of any feature work.

Test plan

• go test -run TestRebase_interactiveRetryPreservesTerminal -count=10 ./internal/git/ — 10/10 passing locally
• Verify on Windows CI runner once this lands

[ed-irl]

This change is part of the following stack:

• test: stabilize TestRebase_interactiveRetryPreservesTerminal on Windows #1227 ◀
  • feat: add worktree management commands, per-worktree trunks, and worktree-aware filtering #1221

_{Change managed by git-spice.}