upgrade: update only requirements if we can
Upgrade operations which involves just upgrading requirements earlier used to go
through whole revlog cloning business.
Now we just upgrade the requirement and skip the cloning part.
Differential Revision: https://phab.mercurial-scm.org/D9775
engine: add `if True` to prepare for next patch
This will help making next patch easier to read.
Differential Revision: https://phab.mercurial-scm.org/D9774