searcher: use DiffSymbols instead of git.Command
Created by: keegancsmith
This runs the same command we are running via git.Command. In the future we should improve the API here. For now this is a minimal change which preserves functionality.
Test Plan: go test as well as manually confirming the arguments are the same in the DiffSymbols implementation.
Part of https://github.com/sourcegraph/sourcegraph/issues/38235