Previously, on 'borg diff', the output always had first the modifications, then
additions, and finally removals. Output may be easier to follow if the various
kinds of changes are interleaved. This commit is a simple solution that first
collects the output lines and sorts them by file path before printing. This new
behavior is optional and disabled by default. It can be enabled with '--sort'
command line option.
This option will be especially useful after the planned multi-threading changes
arrive. Multi-threading may shuffle the archive order of files making diff
output hard to follow without sorting.
Resolves#797.