git-gui: Don't offer to stage hunks from untracked files

git-gui: Don't offer to stage hunks from untracked files

If the user looks at an untracked file in our diff pane we used
to offer "Stage Hunk For Commit" in the context menu when they
right-clicked in that pane.  The problem is we don't actually
have any diff hunks in untracked files, so there is nothing to
really select for staging.  So we now grey out the menu item,
so the user cannot invoke it and think its broken when it does
not perform any useful action.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>