Add git top-level path when adding cscope

[hygoni]

Hello, just made a small change to quickr-cscope.vim. Pick this if you think it is needed.

I was using it to traverse the linux sources, but it was difficult to go to function definitions when I was in subdirectories (like drivers/net/....)

Because the file path of function definition is relative, I was not able to open it in subdirectories. So converted it to absolute path using the git root repository. (Maybe there's better way because there will be a repository that does not use git)