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'