singlehead: making config item a bool again
with the use of `configsuboptions`, the main config item has become
a string (unless it's just the default value).
This makes it in particular hard to override in a cascade of HGRC files,
as we do in Heptapod to re-allow multiple heads on specific repositories
while the default behaviour is to forbid them. The added test case reflects
that use-case
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