Resolves #77.

Update the functional tests to initialize a sparse enlistment with `scalar sparse --add-stdin` and to test lots of Git commands in that mode versus a vanilla Git repo. Uses the SparseMode checks from microsoft/vfsforgit to verify the contents _inside the cone_ match, but ignores all changes outside the cone. 

This took a lot longer than anticipated. Here are a few things that happened along the way:

* The untracked cache doesn't work correctly on Windows. The functional tests were failing due to quickly running status after every call. This _was_ working correctly on Mac, so maybe we re-enable it on a platform-specific basis later. Reverts #86 until we can revisit.

* The clone verb was causing problems. This is due to some timing concerns, mostly: VFS for Git needs a checkout to construct an index before mounting, but we can't checkout before mounting. Since the sparse verb doesn't prefetch before checkout (it doesn't need _everything_, but needs _some_ things), we needed to change this order. However, the `CloneVerb` code is very convoluted. It required a lot of refactoring to split out that checkout code. Hopefully the new code is cleaner and easier to modify.

* The input for the cone matches is slightly different for Git: we need `/` as the path separator, but the logic for interpreting those paths in the test code uses the platform-specific separator.

* The parallel test scripts were causing new issues in the `LooseObjectStepTests`. Turns out that in #68, I messed up the base constructor parameters and didn't give that enlistment a local object cache. That meant that other clones had prefetches and loose object downloads causing different numbers in these tests. That's fixed.

* `CloneWithDefaultLocalCacheLocation()` was failing on the build machine but not on my laptop. The error appeared to be due to failing to register the mount with the service. Turns out, this test used to use the `--no-mount` option, but that was removed in #80. Oops! Maybe we'll put that back as an `--internal_use_only` parameter. OR we can figure out why the service isn't running during that test.