Ignore case when checking the GitError raised
    
    It looks like a future version of libgit2 is going to change the case of
    its error message:
    https://github.com/libgit2/libgit2/commit/909d5494368a00809bc42f4780e86f4dd66e4422
    
    So in order to be forward compatible, let's just ignore the case when we
    perform our check.