Prune deleted branches
When a branch is removed in Git fork :
- that branch should be removed in local Git repository after fetching
- Mercurial bookmark should be deleted
- to discuss: if the bookmark got no child revision, prune the entire branch (how?)
NB: it is assumed that Mercurial bookmarks are only used for pushing Git branches, thus if the Git fork doesn't contain a branch, it can be safely removed in local Git repository (a Git branch being equivalent to a Mercurial bookmark with hg-git).