Merge branch 'jk/diff-no-index-initialize'

"git diff --no-index" may still want to access Git goodies like
--ext-diff and --textconv, but so far these have been ignored,
which has been corrected.

* jk/diff-no-index-initialize:
  diff: reuse diff setup for --no-index case

t/t4053-diff-no-index.sh

@@ -137,4 +137,12 @@ test_expect_success 'diff --no-index from repo subdir with absolute paths' '
 	test_cmp expect actual
 '
 
+test_expect_success 'diff --no-index allows external diff' '
+	test_expect_code 1 \
+		env GIT_EXTERNAL_DIFF="echo external ;:" \
+		git diff --no-index non/git/a non/git/b >actual &&
+	echo external >expect &&
+	test_cmp expect actual
+'
+
 test_done