New diff gatherer based on git diff, works only using additions and deletions
debugging statements, and a fix for the case when adding a line at the end of the file.
Need to use execute to actually unplace signs
Check if b:colourable exists before checking it. 

The BufWinEnter command does not seem to be triggered for some unreadable buffers like ctrlp
Start using git_diff instead of diff. No more notion of a changed line.
Move out the line colouring itself into a separate class.