stack: add some header with the topic name This is the first step of a move to add more data about a stack in the command output.

test: add testing of stack output in 'stack-data' test. We are going to display common information, so we reuse test case.

test: rename 'list' test to 'stack-data' We are about to add stack data information to 'hg stack'. As we'll re-use the test data creating the 'test-topic-list.t' we need to rename it first to embrace its new semantic.

stack: also dim bases Now that they may have index it seems better to have a stronger marking for bases.

stack: reusing the index number in base when applicable This clarify the branching when it is easy to track back the branching point. This does not takes the evolution graph into account yet.

stack: extra argument validation logic outside of showstack It seems sensible keep the 'showstack' function purely about actually displaying and existing stack.