diff: correctly disable external_diff with --no-ext-diff

Upon seeing a type-change filepair, "diff --no-ext-diff" does not
show the usual "deletion followed by addition" split patch and does
not run the external diff driver either.

This is because the logic to disable external diff was placed at a
wrong level in the callchain.  run_diff_cmd() decides to show the
split patch only when external diff driver is not configured or
specified via GIT_EXTERNAL_DIFF environment, but this is done before
checking if --no-ext-diff was given.  To make things worse,
run_diff_cmd() checks --no-ext-diff and disables the output for such
a filepair completely, as the callchain below it (e.g. builtin_diff)
does not want to handle typechange filepairs.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/t/t4020-diff-external.sh b/t/t4020-diff-external.sh
index 083f62d1d6..025df533ca 100755
--- a/t/t4020-diff-external.sh
+++ b/t/t4020-diff-external.sh
@@ -48,7 +48,26 @@ test_expect_success 'GIT_EXTERNAL_DIFF environment and --no-ext-diff' '
 
 '
 
+test_expect_success SYMLINKS 'typechange diff' '
+	rm -f file &&
+	ln -s elif file &&
+	GIT_EXTERNAL_DIFF=echo git diff  | {
+		read path oldfile oldhex oldmode newfile newhex newmode &&
+		test "z$path" = zfile &&
+		test "z$oldmode" = z100644 &&
+		test "z$newhex" = "z$_z40" &&
+		test "z$newmode" = z120000 &&
+		oh=$(git rev-parse --verify HEAD:file) &&
+		test "z$oh" = "z$oldhex"
+	} &&
+	GIT_EXTERNAL_DIFF=echo git diff --no-ext-diff >actual &&
+	git diff >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'diff attribute' '
+	git reset --hard &&
+	echo third >file &&
 
 	git config diff.parrot.command echo &&