config: add some more documentation around why svn and git subrepos are off

subrepo: disable git and svn subrepos by default (BC) (SEC) We have a security issue with git subrepos. I'm not sure if svn subrepo is vulnerable, but it seems not 100% safe to allow writing arbitrary data into a metadata directory. So for now, only hg subrepo is enabled by default. Maybe we should improve the help to describe why git/svn subrepos are disabled.

subrepo: extend config option to disable subrepos by type (SEC) This allows us to minimize the behavior change introduced by the next patch. I have no idea which config style is preferred in UX POV, but I decided to get things done. a) list: 'allowed = hg, git, svn' b) sub option: 'allowed.hg = True' or 'allowed:hg = True' c) per-type action: 'hg = allow', 'git = abort'

subrepo: add config option to reject any subrepo operations (SEC) This is an alternative workaround for the issue5730. Perhaps this is the simplest way of disabling subrepo operations. It does nothing clever, but just aborts if Mercurial starts accessing to a subrepo. I think Greg's patch is more useful since it allows us to at least check out the parent repository. However, that would be confusing if the default is flipped to checkout=False and subrepos are silently ignored. I don't like the config name 'allowed', but I couldn't get any better name.