tests: add systematic test of merge ancestor calculation There is probably some overlap with the existing tests - it is hard to figure out what these tests are doing.

tests: add test case for criss cross merge The test do not fit in any existing test file...

merge with stable

revset: changed generatorset code to remove unnecesary function call Removed _nextitem() method, now __iter__ has that logic and __contains__ uses __iter__ to check for membership.