Merge branch 'sb/diff-orderfile-config'

Allow "git diff -O<file>" to be configured with a new configuration
variable.

* sb/diff-orderfile-config:
  diff: add diff.orderfile configuration variable
  diff: let "git diff -O" read orderfile from any file and fail properly
  t4056: add new tests for "git diff -O"

diff --cc diff.c
Simple merge

diff --cc diffcore-order.c
Simple merge