histedit: move constraint verification to the 'action.verify' method
Action has a method dedicated to verifying its validity. So we move code
related to constrains into that method. This requires a bit more context to the
'verify' method in the same fashion we were passing the 'prev' argument.
This is an extra step before we can simplify the constraint handling code
further.
% creating repo
% add and commit
C test.py
% change
M test.py