cmd/gitserver: Move doClone into it's own method

Created by: indradhanush

This is the first of a series of steps to reduce goroutine creation for each new clone.

The doClone closure is very involved and while having it defined withing cloneRepo allows us to not have to worry about argument passing, it makes understanding the cloneRepo method itself laborious. As a result, moving it to a method is a good trade off.