SparseVerb: the progress in 'git read-tree' is not passed through to user

[derrickstolee]

Deep down in GitProcess.InvokeGitImpl(), we parse the stderr output from our Git process. This means a user will not see the progress put into git read-tree (via git sparse-checkout) in microsoft/git#200.

Either

1. allow pass-through of this progress or
2. use ShowStatusWhileRunning() to do a spinning wheel over this process.

The issue is that the git read-tree output actually shows a numerical progress indicator, so a user can see how much time is left, roughly.

[derrickstolee]

We are getting rid of the sparse verb in #122, so this is not needed.