test(repo sync): navigation history for unsubmitted branches

Add a test for how merge history propagation behaves
for upstack branches that are unsubmitted.

Suppose I have `feat1 -> feat2`:

- feat1 is ready and I submit a CR
- feat2 is not yet ready for review, so it is unsubmitted
- while I'm still working on feat2, feat1 gets approved and I merge it
- I then submit feat2

Right now--by accident--it works so that feat2 receives the history:
it will show feat1 as its merged downstack even though feat1 was merged
by the time feat2 was restacked on main and submitted.

My intuition is that this should NOT be supported,
but it's also coming mostly for "free".

I'm opening up this PR and test case to discuss this behavior
with anyone who cares.

testdata/script/repo_sync_unsubmitted_upstack_history.txt

@@ -0,0 +1,71 @@
+# Merge history propagation when a branch is submitted and merged,
+# while its upstacks are still unsubmitted.
+
+as 'Test <[email protected]>'
+at '2024-12-21T12:24:48Z'
+
+# set up
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+gs repo init
+
+# set up a fake GitHub remote
+shamhub init
+shamhub new origin alice/example.git
+shamhub register alice
+git push origin main
+
+env SHAMHUB_USERNAME=alice
+gs auth login
+
+# set up main -> feat1 -> feat2
+git add feat1.txt
+gs branch create feat1 -m 'Add feature 1'
+git add feat2.txt
+gs branch create feat2 -m 'Add feature 2'
+
+# Submit and merge feat1
+gs bottom
+gs bs --fill
+stderr 'Created #1'
+shamhub merge alice/example 1
+gs rs
+gs sr
+
+# Submit feat2
+gs bco feat2
+gs ss --fill
+stderr 'Created #2'
+
+gs ls
+cmp stderr $WORK/golden/ls-after.txt
+
+shamhub dump comments
+cmp stdout $WORK/golden/final-comments.txt
+
+-- repo/feat1.txt --
+feat 1
+-- repo/feat2.txt --
+feat 2
+-- golden/ls-after.txt --
+┏━■ feat2 (#2) ◀
+main
+-- golden/final-comments.txt --
+- change: 1
+  body: |
+    This change is part of the following stack:
+
+    - #1 ◀
+
+    <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
+    <!-- gs:navigation comment -->
+- change: 2
+  body: |
+    This change is part of the following stack:
+
+    - #1
+        - #2 ◀
+
+    <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
+    <!-- gs:navigation comment -->