Force pushing to a cog repository breaks Downloader's update functionality

### What component of Red (cog, command, API) would you like to see improvements on?

Downloader

### Describe the enhancement you're suggesting.

When you force push to a repo and then attempt to update that repo or a cog within it, you get an error the first time and then silent failures on future attempts. Instead, Red should look at the original repo link and re-fetch the latest commit on that branch.

### Anything else?

I've mentioned this a few times before in Discord and I think @jack1142 had it planned, but figured I should make a trackable issue.