--- a/tests/test-extdiff.t	Tue Jun 20 09:33:19 2017 -0400
+++ b/tests/test-extdiff.t	Thu Jun 15 14:27:52 2017 -0400
@@ -249,7 +249,7 @@
 Prepare custom diff/edit tool:
 
   $ cat > 'diff tool.py' << EOT
-  > #!/usr/bin/env python
+  > #!$PYTHON
   > import time
   > time.sleep(1) # avoid unchanged-timestamp problems
   > file('a/a', 'ab').write('edited\n')