test: issue a clarifying graph print in tests/test-stack-branch.t

stack: properly order stack when gaps existing inside it We transitively search for the next "stack" ancestors, of changeset in the stack not based on other revision of the stack. This should help having a consistent display when topic are interleaved.

stack: display both parent with displaying merge This will help understanding merge in the stack. More test involving merge are adding in the next changeset.

tests: add tests for showing stack when merge is involved This patch adds tests of showing stack when merges are involved. This will help us in changing the behaviour and realising that in later patches.

stack: move data computation on the object We gather multiple data about the stack on the object. Let us move this on the object and keep it cached there.

stack: replace 'getstack' with direct call to 'stack' The dedicated function do not add any value. We can update all the callers.

stack: introduce a rich stack object The logic around stack object become more and more complex. We great and object to gather and abstract it.

tests: add tests for `hg stack` showing surprising behavior with gaps The test will help us to analyse in next commit if the things are fixed or not.

topic: check availability of obsutil.getmarkers() for portability Before this patch, topic extension causes unintentional failure with Mercurial earlier than 4.3, because obsutil.getmarkers() has been available since Mercurial 4.3 (tests for topic on mercurial-4.* branches fail, too). This breaks "minimumhgversion = '4.0'" declaration of topic extension. This patch fixes this issue in a straightforward way for simplicity on stable branch. I'm planning to centralize such portability logic in topic extension into topic/compat.py or so on default branch for efficiency at runtime, like as evolve/compat.py.

tests: add more explicit test about empty topic from publishing