vfs: add dirname

subrepo: use vfs.basename instead of os.path.basename

vfs: add basename

subrepo: use repo.pathto instead of util.pathto to simplify invocation This centralization into 'repo.pathto()' should reduce the cost of vfs migration around 'getcwd()' and so on in the future.

subrepo: prefetch ctx.repo() for efficiency and centralization 'subrepo.state()' refers same 'ctx.repo()' in many places and times.

revset: rename getkwargs() to getargsdict() This function was added recently at 48919d246a47, but its name was misleading because it processes both positional and keyword arguments.