[Downloader] Repair repo function

# Feature request

#### Select the type of feature you are requesting:

- [ ] Cog
- [x] Command
- [x] API functionality

#### Describe your requested feature

I discussed a bit in #coding about a command in Downloader to repair broken repos. *This may be related to the nukes recently dropped in my repo.*

I'd gladly work on it, but I feel like we need to discuss about what it exactly does before I start coding. There are some questions that needs to be discussed, such as:
- Should the repair function simply remove and readd the repo? It could run other commands, there are plenty of commands made for repair (such as `reset`, `prune`, `fsck`, `gc` or `reflog`), not sure if it could be useful
- Should it attempt to repair cogs? If I take the example of my recently broken repo, people had to remove and readd the repo, then uninstall and install the `codmw` cog, other cogs were fine. How could the bot detect what cogs are broken and should be reinstalled?