Tabstaff: Epmty String no longer prefered when accord spans 2 octaves

[s . runkel]

Hi, 

Just upgraded from 2.12 to 2.14.2.

Problem:
If an accord spans more than or equal to 2 octaves,
an empty string is not used but the lowest string shows a fretted position.

Example:
  \score {
        \new TabStaff {
          < a, gis' >1
          < a, a' >1
          < a, ais' >1
          < a, dis a' >1
        }
  }

results in:
---4---5---6---5---
-------------------
-------------------
---------------1---
---0---------------
-------5---5---5---

But should read as in 2.12:
---4---5---6---5---
-------------------
-------------------
---------------1---
---0---0---0---0---
-------------------

Example shows that:
- it happens exactly at an interval of >= 2 octaves
- it has nothing to do with barree