Fix test reporting when in interactive mode.

When runing in interactive mode, the diff between expected and output is not being show,
Supposedly, Git-diff returns 0 on successful run and a non-zero for error.
However, this is not being picked up by the test suite.

We change git-diff's error reporting to be more diff like in which:

+ +ve indicates that there is a diff.
+ 0 indicates the files are the same.
+ -ve program error.

This should make diff reporting more robust.

tests/Main.idr

@@ -199,8 +199,8 @@ runTest opts testPath
               ]
             Just exp => do
               putStrLn "Golden value differs from actual value."
-              code <- system "git diff expected output"
-              when (code /= 0) $ printExpectedVsOutput exp out
+              code <- system "git diff --exit-code expected output"
+              when (code < 0) $ printExpectedVsOutput exp out
               putStrLn "Accept actual value as new golden value? [yn]"
           b <- getAnswer
           when b $ do Right _ <- writeFile "expected" out