tests/test-ancestor.py.out

help: revise explanation about capability check while selecting merge tool

This is follow up of 7c6044634957 and cded904f7acc.

This patch adds explanations about:

  - notation in capability columns in the table
  - how capabilities of external merge tools are treated

% lazy ancestor set for [], stoprev = 0, inclusive = False
membership: []
iteration:  []
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = False
membership: [7, 8, 3, 4, 1, 0]
iteration:  [3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [1, 3], stoprev = 0, inclusive = False
membership: [1, 0]
iteration:  [0, 1]
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = True
membership: [11, 13, 7, 8, 3, 4, 1, 0]
iteration:  [11, 13, 3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = False
membership: [7, 8]
iteration:  [7, 8]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = True
membership: [11, 13, 7, 8]
iteration:  [11, 13, 7, 8]