changegroup: stop returning and recording added nodes in 'cg.apply'
cg.apply used to returns the added nodes. Callers doesn't have a use for it
anymore, remove the added node and stops recording it in the current
operation.
This information was added in the current release cycle so no extensions
breakage should happens.
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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273ce12ad8f1` will report some style violations on a very old commit).