topic: centralize compatibility logic between hg versions into compat module This patch can delay loading obsutil and obsolete modules until they are actually used at "hg topics" or so, if demandimport of Mercurial is enabled.

readme: mention the improvement to stack in the readme

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.