patman: Modify functional tests for new behavior

This patch adds or modifies functional tests for the Cover-changes,
Commit-changes, and Series-process-log tags in order to account for new
behavior added in the previous few patches. The '(no changes since v1)'
case is not tested for, since that would need an additional commit to test
in addition to testing the existing code paths.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>