flake8: fix E127 style

flake8: fix W391 style

tests: add flake8 check This test will hopefully help prevent style errors. To be determined: which errors / warnings to ignore?

flake8: add file for rules to ignore This will help prevent users that have an editor with flake8 capabilities from fixing linter errors for rules that are not wanted by the project. Initially, this ignore list includes everything in the project that violates pep8. I'll try to ween this list down to something a little more manageable.

init: guard _namemap with repo.topics (issue5351) This prevents reading extra data on all repo commits when there is no topic for a given name. _namemap is called a lot, and it is often called for commit shas -- which should pretty much never be names of topics...

init: cache repo.topics

stack: add warning about multiple heads This seems a good spot to nudge people toward reducing there head count.

stack: add some behind information This is the next useful thing after branch

stack: add some basic branch information In the same spirit as knowing the topic you are on, having some branch data seem useful (yet, this smell a bit like summary)

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.