bundle2: fix bundle2 pulling all revs on empty pulls
When bundle2 was enabled, if hg pull had no commits to pull, it would print
'no changes found' and then download the entire repository from the server. This
was caused by heads and common being set to None, which gets treated as
heads=cl.heads() and common=[nullid], which means download the entire repo.
Pulling bundles without a changegroup is a valid use case (like if we're just
updating bookmarks), so this modifes the bundle code to allow not adding
changegroups.
This is backport of
ab5040cd5749.
workingfilectx.date = (1000, 0)
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