We solve that problem using naming conventions. Branch names must start with the issue key (we use Jira). You don't do anything in that branch that's not part of that issue. If you do, you must prefix the commit message with the issue key that it goes with. The commit itself identifies what changed. The Jira issue provides all the backstory and links to any supporting materials (design docs, support tickets, etc). I have to do a lot of git archeology in my role, and this scheme regularly allows me to figure out why a code change was made years ago without ever talking to anyone.