Git status refinements and support vfs/gvfs repos

[DefaultRyan]

Summary of the pull request

• Re-use and cache command-line git repo status for individual files.
• • This is so we no longer have any "status" calls to LibGit2Sharp that could trigger a filesystem traversal to inadvertently over-hydrate the repo. Remaining libgit2sharp calls should not traverse the file tree for status, and any status calls will go through the command-line which is enlightened about VFS/GVFS.
• Use FileSystemWatcher to look for file changes that could invalidate the cached status.
• Throttle file system events to not flood the system with command-line status invocations.
• Notify the Shell when a file status changes.

References and relevant issues

Detailed description of the pull request / Additional comments

There's some code movement here to move GitRepositoryStatus and GitStatusEntry from RepositoryWrapper.cs to their own files.

Introduced StatusCache which caches the most-recently obtained status and queues status updates in response to filesystem changes in the repo.

These updates are throttled so that if multiple file events arrive rapidly, we don't fetch another status update until the prior one has completed and the waiting period expires (currently 3 seconds).

Validation steps performed

Built a private DevHome build.

Verified that file status still appears as expected. Verified that file and repo status both eventually report the correct status after making multiple rapid file changes.

Attached a debugger with printing breakpoints to validate that even when making multiple rapid file changes, the status is only updated after a 3 second delay.

PR checklist

• Closes #xxx
• Tests added/passed
• Documentation updated