Fix going ahead of the history of the file turning screen blank forever.

When pressing back (`[`) until before the commit that added
the inspected file, the blame contents go blank (makes sense).

When going forward again, until now it was possible that they
stay blank because `highlight_line` could become `-1`, and
once this happened, it would stay `-1` forever.

This commit fixes it by making sure that `max_highlight`
cannot be negative.

Author: nh2

gitbrowse/ui.py

@@ -81,8 +81,8 @@ def highlight_line(self):
 
     @highlight_line.setter
     def highlight_line(self, value):
-        # Ensure highlighted line in sane
-        max_highlight = len(self.file_history.blame()) - 1
+        # Ensure highlighted line is sane
+        max_highlight = max(0, len(self.file_history.blame()) - 1)
         if value < 0:
             value = 0
         elif value > max_highlight: