incoming: don't request heads that already are common
Pull would send a getbundle command where common heads were sent both as common
and head, even though there is no reason to request a common head.
The request was thus twice as big as necessary and more likely to hit HTTP
header size limits.
Instead, don't request heads that already are common.
This is fixed in bundlerepo.getremotechanges . It could perhaps also have been
fixed in discovery.findcommonincoming but that would have a bigger impact.