Fix regression from "Improve staging of diff chunks"

Fix regression from "Improve staging of diff chunks"

In commit 234918423a4fac44375317e6c9e7ee54ea09379c the change to
stage_update reads:

 static bool
 stage_update(struct view *view, struct line *line)
 {
-       if (!opt_no_head && stage_line_type != LINE_STAT_UNTRACKED &&
-           (line->type == LINE_DIFF_CHUNK || !stage_status.status)) {
-               if (!stage_update_chunk(view, line)) {

where the !stage_status.status part was meant to match whole sections.
Reinstate this behavior so that pressing 'u' when in the stage view for
a section of changes it will stage all changed files in that section.