Improved experience for displaying git prompt status

Using cmder when working with large git repositories can be non-ideal experience due the fact running git status takes a while which results in prompt display to be very slow.

This change makes changes to cmder to use gitprompt tool instead of git to display prompt.
gitprompt (https://github.com/fieryorc/gitprompt) provides cached status for git repositories, which has much better experience on larger repos.

Also I have published a release based on this change, if you want to test.
https://github.com/fieryorc/cmder/releases/tag/v1.2-gitprompt.1