vcsdiff: use (cleared) augroup for autocommands

[blueyed]

This allows for proper handling when resourcing the file.

[tomtom]

What's the idea behind putting every single autocmd into a group of its own? Could you please explain the problem. Thanks.

[tomtom]

Hi! What's the idea behind putting every single autocmd into a group of its own?

[blueyed]

It helps when resourcing the file.
It does not need to use different augroups, but IIRC the main one is also used in another file, that's likely why I've used separate ones.