[m-rev.] for review: Produce unified diffs for test cases by default.

[Peter Wang]

tests/Mmake.common:
    Set DIFF_OPTS=-u if not defined.

diff --git a/tests/Mmake.common b/tests/Mmake.common
index 974a8868c..f870c793e 100644
--- a/tests/Mmake.common
+++ b/tests/Mmake.common
@@ -40,7 +40,7 @@ endif
 # file by setting both GRADEFLAGS-foo.
 
 ifndef DIFF_OPTS
-DIFF_OPTS=-c
+DIFF_OPTS=-u
 endif
 
 # Override this with `RUN_RECOMPILATION_TESTS=no' if your compiler
-- 
2.21.0