[hg] Per the TODO on HgWorkingCopyRevisionsCommand::parents(), that meth...

...od now returns a Pair, matching all the other versions of the parents() method.

I had a really hard time getting the org.zmlx.hg4idea.test.HgUpdateTest#updateShouldMergeButNotCommitWithConflicts to work. I added a bunch of comments explaining what each step of the test does and what the resulting history is supposed to be like. I did not make any attempt at getting the other tests in this class to work.