search backend: factor out commit search comparator

Created by: rvantonder

I need a function that accepts a []SearchResultResolver type and sorts commits, see the use up the stack at #16641. compareSearchResults is unsuitable because it doesn't have any commit logic, and after trying to make it work, got frustrated with the searchResultURIGetter signature and stopped.

Instead, am factoring out the comparator to a function I can call directly for data containing commit results.