Adjust the order mergetools feeds the files to the p4merge backend
to match the p4 convention.

* kb/p4merge:
  merge-one-file: force content conflict for "both sides added" case
  git-merge-one-file: send "ERROR:" messages to stderr
  git-merge-one-file: style cleanup
  merge-one-file: remove stale comment
  mergetools/p4merge: create a base if none available
  mergetools/p4merge: swap LOCAL and REMOTE