Merge branch 'jv/maint-no-ext-diff'

"git diff --no-ext-diff" did not output anything for a typechange
filepair when GIT_EXTERNAL_DIFF is in effect.

* jv/maint-no-ext-diff:
  diff: test precedence of external diff drivers
  diff: correctly disable external_diff with --no-ext-diff

diff --cc diff.c
Simple merge

diff --cc t/t4020-diff-external.sh
Simple merge