Merge branch 'jc/ll-merge-expose-path'

Merge branch 'jc/ll-merge-expose-path'

Traditionally, external low-level 3-way merge drivers are expected
to produce their results based solely on the contents of the three
variants given in temporary files named by %O, %A and %B on their
command line.  Additionally allow them to look at the final path
(given by %P).

* jc/ll-merge-expose-path:
  ll-merge: pass the original path to external drivers