Add 'tracked' context to git-info

[Zoybean]

Motivation

Knowing when the working tree is dirty is useful, but when the only dirty files are untracked, the tree could be considered 'cleaner' than if tracked files had changed, because you are still able to perform a git rebase or git pull, or change branches freely. This information could be useful to know at a glance, so it would be good to be able to display the number of changes to tracked files in the prompt.

Proposed Changes

Add a 'tracked' context to git-info corresponding to the number of tracked files which are 'dirty'.