gitstats: Improved the test descriptions

gitstats: Improved the test descriptions

The test descriptions were not very coherent. Instead of
following a specific format, most tests didn't follow any
description format at all. The current description format
is as follows:
'Test that "function [<options>]" [works|filters] as
expected [for <specific case>]'
In the case that a test is only there to point out an
untested option, the message should be as follows instead:
'There is no test for "function [<options>]"'