Treat github repository URLs as case-insensitive

When installing swiftformat using `mint install swiftformat`, I got this:

```bash
There are multiple git repositories matching 'swiftformat', which one would you like to use?
  1. https://github.com/nicklockwood/SwiftFormat.git
  2. https://github.com/nicklockwood/swiftformat.git
```

I don't recall ever seeing this before in mint, but it doesn't seem very useful.