narrow: build the known set of nodes only when ellipses is enabled
We don't need to build the known set in non-ellipses case because we don't have
a shallow repo. In this patch, this checks whether the server has ellipses
enabled or not using the server capability and then build the known set of
nodes.
Building the known set of nodes can take ~3-4 minutes on repositories with
millions of csets so this patch speeds up extending a non-shallow narrow clone
on large repositories.
In future, we should first check whether local repository is an ellipses repo
using a new ellipses repo requirement and then control all the combinations
between local repo requirement and server capability.
Differential Revision: https://phab.mercurial-scm.org/D4520