libmain/progress-bar: actually don't show short activities

The progress bar is not supposed to show very short activities (<10ms) in order
to prevent excessive flickering. This behaviour was inadvertedly disabled in
commit da4e46dd1f due to not actually skipping
the short activity after calculating that it should have been. Fix this
oversight.

Fixes: #561
Change-Id: Ibee256a7018cdc431662cd8ea9f6c14ef1706972

src/libmain/progress-bar.cc

@@ -382,6 +382,7 @@ std::chrono::milliseconds ProgressBar::draw(State & state, const std::optional<s
                         delay - (now - i->startTime)
                         )
                     );
+                continue;
             }
 
             activity_line = i->s;