discovery: avoid wrongly saying there are nothing to pull
We can get in a situation where a revision passed through `hg pull --rev REV`
are available on the server, but not a descendant of the advertised server
heads.
For example the server could lying be during heads advertisement, to hide some
pull request. Or obsolete/hidden content could be explicitly pulled.
So in this case the lookup associated to `REV` returned successfully, but the
normal discovery will find all advertised heads already known locally. This flip
a special boolean `anyinc` that will prevent any fetch attempt, preventing `REV`
to be pulled over.
We add three line of code to detect this case and make sure a pull actually
happens.
My main target is to make some third party extensions happy (I expect the
associated test to move upstream with the extension). However this fix already
make some of the `infinitepush` test happier.
# See http://EditorConfig.org for the specification
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