diff --git a/t/test-lib.sh b/t/test-lib.sh
index f59b34a824..2e6b516586 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -775,6 +775,8 @@ export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR GIT_CONFIG_NOSYSTEM GIT_CONFIG_NOGLOB
 
 . ../GIT-BUILD-OPTIONS
 
+DIFF="${DIFF:-diff}"
+
 if test -z "$GIT_TEST_CMP"
 then
 	if test -n "$GIT_TEST_CMP_USE_COPIED_CONTEXT"