Fix test reporting when in interactive mode.
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.

mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm