Add gui.displayuntracked option

When git is used to track only a subset of a directory, or
there is no sure way to divide files to ignore from files to track,
git user have to live with large number of untracked files. These files
present in file list, and should always be scrolled through
to handle real changes. Situation can become even worse, then number
of the untracked files grows above the maxfilesdisplayed limit. In the
case, even staged can be hidden by git-gui.

This change introduces new configuration variable gui.displayuntracked,
which, when set to false, instructs git-gui not to show untracked files
in files list. They can be staged from commandline or other tools (like
IDE of file manager), then they become visible. Default value of the
option is true, which is compatible with current behavior.

Signed-off-by: Max Kirillov <max@max630.net>
Squashed