Coding Agent comments about non-existent commit

[jonathanpeppers]

Describe the bug

You can see the comment here:

• Stable Updates 20253006 - Update Maven dependencies and documentation dotnet/android-libraries#1192 (comment)

Copilot has successfully completed its task, but it says:

70fcd59 is a non-existent commit, note that it isn't rendered as a hyperlink.

To Reproduce
I'm not sure how to reproduce exactly.

In this case, this happened:

• Assigned Copilot an issue, it opened an initial PR that needs more iteration.
• Commits were merged into main
• I clicked the "Update Branch" button to get latest main
• Copilot setup steps were broken, due to changes in main
• Fixed by a new commit in main
• I clicked the "Update Branch" button to get latest main, again
• I commented @copilot try again, basically
• Copilot did its work and mentioned the weird git sha.

Expected behavior
The comment should mention 6d16609, which is the commit Copilot actually pushed to the PR.