Filter changed file lists

[kuychaco]

As requested in #845 to make working with lots of files easier

[BinaryMuse]

From #1131 by @vpacik

Currently the Unstaged Changes window displays list of all files (excepts one filtered out by .gitignore) - both tracked and untracked - which can make it quite long in same cases.

Suggestion: make a option to filtered out all untracked files and keep only the tracked ones similar to
git status --untracked-files=no or globally set git config status.showuntrackedfiles no directly in the window or in the package settings.