navigation comment: merged CR history limit

When we post the navigation comment for a branch in the stack, it includes the full downstack history for all known downstack branches.
For long-lived stacks, this can result in deeply nested lists which GitHub (and probably other systems) aren't good at rendering. Beyond 10, all items are rendered in the same itemized list without topology represented.
We should include a configuration option to limit how far back the merged downstack history goes.

Maybe we can overload [spice.submit.navigationComment.downstack](https://abhinav.github.io/git-spice/cli/config/#spicesubmitnavigationcommentdownstack), which currently has options 'all' and 'open' to also accept numbers for an explicit maximum count of downstack CRs to include.