git-spice tracks local main instead of origin/main

I would like git-spice to be smarter about comparing to `origin/main` instead of my local `main` branch because local `main` is not always up-to-date.

Very possible I've misconfigured `git-spice`, or missed an important step of the flow. Advice is appreciated, thank you.

Steps to reproduce:
I'm using GitHub and `git-spice 0.19.0`.

1. create a stack. For example:
```
❯ gs log long
┏━■ branch-2-bar ◀
┃ cbc5e11 fantastic feature (19 hours ago)
┏━┻□ branch-1-foo
┃ a8b8e11 beautiful bugfix(40 minutes ago)
main
```
2. `git checkout branch-2-bar` Be on top of the stack
3. Run `git fetch` to get the latest `origin/main`. At this point my local `main` branch is behind `origin/main`.
4. Run `gs stack submit`.

Expected behavior:
Pull requests are created for `branch-1-foo` and `branch-2-bar` only.

Actual behavior:
`git-spice` starts to submit changes by describing commits that are already in `origin/main` but not my local `main`.

The proposed PR title comes from a commit on `origin/main`
```
❯ gs stack submit
Title: thing done and landed by a colleague (#12345)
```
so does the proposed PR body.

After I edit the proposed title and body then the commits and files changed in the created PR accurately reflects the commits on each branch.

This could therefore be a problem only with the guessed title and body.