upgrade: rename finddeficiences() to find_format_upgrades()
It was not obvious what does deficieny means and every format upgrade can't be a
deficiency. There are some format upgrades like compression levels, share-safe
which can't be understood at deficiencies.
A change can be an upgrade or downgrade, however this finddeficiences() only
used to find upgrades.
This patch renames the function and related variables to make things more
clearer. The ui message also got improved which is a good thing.
Next patch will rename deficiency in general across the upgrade code.
Differential Revision: https://phab.mercurial-scm.org/D9582