censor: extract the part about recomputing delta in a function
The v2_censor function is huge, now that its content has settled a bit it is a
good time to split individual part inside dedicated function.
It is now the turn of the logic that recompute new delta to replace the one
based on the revision that are going away.
Differential Revision: https://phab.mercurial-scm.org/D10900
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