git-gui: Make F5 reselect a diff, if an untracked file is selected.

git-gui: Make F5 reselect a diff, if an untracked file is selected.

If an untracked file is selected, F5 and other manual rescan synonyms
would try to select a tracked file instead. Also, clicking on an icon
in the unstaged changes list skips over untracked files, unless the
file clicked is untracked itself.

The objective is to make it easier to ignore untracked files showing
up in the Unstaged Changes list, and ensure that no modifications
to tracked objects are left unstaged.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>