10_test_get_upstream_tree: Fix test descriptions
Silence GitRepository.has_treeish

This also silences 10_test_get_upstream_tree
Silence GitRepository.set_upstream_branch

It's not used within gbp itself so no command line tool expects
an error message to be printed.
Silence GitRepository.grep_log

None of the gbp script rely on the output being printed. In fact we
improve the exception by appending the stderr output.