diff --git a/t/test-lib.sh b/t/test-lib.sh
index 75ddb7abb5..3b08a38655 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -76,6 +76,8 @@ export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME
 export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
 export EDITOR
 
+DIFF="${DIFF:-diff}"
+
 if test -z "$GIT_TEST_CMP"
 then
 	if test -n "$GIT_TEST_CMP_USE_COPIED_CONTEXT"