Print "git diff" output when the checksums tests has failed (#306)

In some cases this output is useful to identify the root of the problem.
See https://github.com/postmarketOS/pmbootstrap/pull/303#issuecomment-319017197

test/check_checksums.py

@@ -46,6 +46,8 @@ def check_checksums(package):
     if result == "":
         print("** The checksums are correct")
     else:
+        print(result)
+        result = check_output_always(['git', 'diff']).decode()
         print(result)
         print("** The checksums are not correct")
         exit(1)