bookmark: track bookmark changes at the transaction level
The transaction has now a 'bookmarks' dictionary in tr.changes. The structure
of the dictionary is {BOOKMARK_NAME: (OLD_NODE, NEW_NODE)}. If a bookmark is
deleted NEW_NODE will be None. If a bookmark is created OLD_NODE will be None.
If the bookmark is updated multiple time, the initial value is preserved.
{
"version": 1,
"project": "mercurial",
"project_url": "https://mercurial-scm.org/",
"repo": "..",
"branches": ["default", "stable"],
"environment_type": "virtualenv",
"show_commit_url": "https://www.mercurial-scm.org/repo/hg/rev/",
"benchmark_dir": "benchmarks",
"env_dir": "../.asv/env",
"results_dir": "../.asv/results",
"html_dir": "../.asv/html"
}