Add options to restrict data showing in status view

[Avinash-Bhat]

This is to essentially prevent the forever-loading scenario of the status info.

This is apparent if one use vcsh for dotfile management.
Since it uses the user's home directory as the working directory, it takes forever to load the status screen.

In this scenario, it's better to allow switches from the git-status command, like -u (a.k.a --untracked-files=) and even going a bit further, and reading the default from status.showUntrackedFiles of git config.

[jonas]

Tig is using the ancient git-update-index, git-diff-files and git-ls-files and not git-status, however, I ended up adding a custom way to not show untracked files.