Merge branch 'jl/maint-diff-ignore-submodules'

* jl/maint-diff-ignore-submodules:
  t4027,4041: Use test -s to test for an empty file
  Add optional parameters to the diff option "--ignore-submodules"
  git diff: rename test that had a conflicting name

diff --cc Documentation/diff-options.txt
Simple merge

diff --cc diff.c
Simple merge