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
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