checkheads: simplify the structure build by preprocessing All useful data are now gathered in a single dictionnary. `branchmapsummary` is renamed to `headssummary` and its return value is greatly simplified.

_updatebranchcache: prevent deleting dict key during iteration We use dict.keys() to fetch all keys before starting to delete some. Otherwise python complains that the dictionnary is altered during iteration.

checkheads: extract branchmap preprocessing The checkheads function is far too complicated. This extract help to explicite what part of the preprocessing are reused by the actual check. This the first step toward a wider refactoring.