submit: Invalidate template cache if origin/main is updated

[abhinav]

We calculate the template cache key based on the hashes of the files inside the locations in the repository where tempates are stored, but we check their state at $trunk instead of $remote/$trunk.

This results in the cache not being invalidated if the remote has been updated and fetched, but the local ref is behind.

Ref #369

[abhinav]

This change is part of the following stack:

• 370 ◀

_{Change managed by git-spice.}