run-tests.py: fix diff output when test-foo.out doesn't exist.
Previously, if test-foo.out doesn't exist and test-foo outputs
something, the first line of the diff will look like this:
-+changeset: 0:0acdaf898367
(note the leading "-")
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -244,7 +244,7 @@ def run_one(test):
ref_out = splitnewlines(f.read())
f.close()
else:
- ref_out = ['']
+ ref_out = []
if out != ref_out:
diffret = 1
print "\nERROR: %s output changed" % (test)