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'
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