hgwebdir: add support for explicit index files This is useful for when repositories are nested in --web-conf, and in the future with hosted subrepositories. The previous behavior was only to render an index at each virtual directory. There is now an explicit 'index' child for each virtual directory. The name was suggested by Yuya, for consistency with the other method names. Additionally, there is now an explicit 'index' child for every repository directory with a nested repository somewhere below it. This seems more consistent with each virtual directory hosting an index, and more discoverable than to only have an index for a directory that directly hosts a nested repository. I couldn't figure out how to close the loop and provide one in each directory without a deeper nested repository, without blocking a committed 'index' file. Keeping that seems better than rendering an empty index.