dirs: remove mutable string optimization at all
As far as I can see, the optimization trick has been dead since 42e89b87ca79
"dirs: speed up by storing number of direct children per dir". After
42e89b87ca79, the key variable is cleared to NULL at each iteration.
{
"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"
}