An executable-bit only change creates invalid properties diff

[ColMelvin]

Sample Output

Index: file
===================================================================

Property changes on: file
___________________________________________________________________
Added: svn:executable
   + *

Pre v1.7
The Index: and === lines should be omitted.

v1.7 and later
After the === lines, the normal --- & +++ lines should be added (but the diff should still have no content).

The solution will likely interact with #19.