Improve displaying status using gitprompt tool.

[fieryorc]

Change description:

1. When doing Get-GitStatus, run gitpromptclient.exe instead of running git.exe.
2. GitPromptClient talks to GitPromptCache.exe to retrive the cached result (It may take same amount of time as 'git status' for first time).
   3, During install, posh-git will download the binaries and store it under bin directory.

Gitprompt tool is available here https://github.com/fieryorc/gitprompt

Advantages:

1. The cache is maintained in a seperate process, and it serves as a cache for multiple powershell instances.
2. It is C++ process using libgit2, so it is lean and mean, also very fast.

[dahlbyk]

I had been toying with the idea of building some sort of shared status watcher/cache for a good year now, but never actually did anything about it - very glad to see that you did!

Rather than bake this in as the default behavior, what if we were to turn GitPrompt into its own module on which posh-git would have an optional dependency? If the module exists, posh-git would use its Get-GitStatus implementation (or some well-defined interface); otherwise, it would retain the existing behavior.

[dahlbyk]

And correct me if I'm wrong, but doesn't the change of, say $status.Index.Added, from a list of paths to just a Count break tab completion on git add <tab> and such?

[fieryorc]

I think that makes sense. I'm rather new to powershell, and I missed that branch list. I will make a new request with the suggested changes.

[dahlbyk]

Feel free to reopen and just push new changes to your branch.