Clear up the semantics of Series.new_patch

This patch adds a number of assertions to document and verify the
complex restrictions of the input parameters to the Series.new_patch
function. It also adds the requirement that 'before_existing' and
'commit' cannot be true at the same time when calling it, instead of
updating 'commit' inside the function.

Signed-off-by: David Kågedal <davidk@lysator.liu.se>
Signed-off-by: Karl Hasselström <kha@treskal.com>