checkheads: extract bookmark computation from the branch loop No branch specific data are used in the computation of the bookmarked heads. We can only compute it once.

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.