dashboard/app: file coverage is not always available

[tarasmadan]

Describe the bug
https://raw.githubusercontent.com/torvalds/linux/7cb1b466315004af98f6ba6c2546bb713ca3c237/virt/kvm/kvm_main.c returns 429 for some reason even when I'm trying to open the link using my browser.
It seems to be the throttling.
GitHub limit is 5k requests/hour per IP.

Expected behavior
Coverage data is always available.

Additional context
Direct links to github are the temporary solution. We're working on syzbot internal storage.
This bug may increase the internal work priority.

[tarasmadan]

Current options:

1. SSM supports file operations. We can't switch to SSM based repo because of the auth problems.
2. https://github.com/go-git/go-git combined with git config core.sparseCheckout true.
3. Use kernel.googlesource.com instead of raw.githubusercontent.com . Example - https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux.git/+/7cb1b466315004af98f6ba6c2546bb713ca3c237/virt/kvm/kvm_main.c .

Option 3 looks the best. Thanks @melver for it.