Merge branch 'rs/external-diff-with-exit-code'

The "--exit-code" option of "git diff" command learned to work with
the "--ext-diff" option.

* rs/external-diff-with-exit-code:
  diff: fix --exit-code with external diff
  diff: report unmerged paths as changes in run_diff_cmd()

t/t4046-diff-unmerged.sh

@@ -98,4 +98,12 @@ test_expect_success 'diff --stat' '
 	test_cmp diff-stat.expect diff-stat.actual
 '
 
+test_expect_success 'diff --quiet' '
+	test_expect_code 1 git diff --cached --quiet
+'
+
+test_expect_success 'diff --quiet --ignore-all-space' '
+	test_expect_code 1 git diff --cached --quiet --ignore-all-space
+'
+
 test_done