WIP - Add ability to preserve items

First stab at adding a preserve-missing-items flag to alter the sync behaviour.

added 1 commit

• 1ebef502 - Add argument to get drush test passing

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added 22 commits

• 1ebef502...ae718137 - 21 commits from branch project:6.0.x
• 13f55cc3 - Merge branch '6.0.x' into 'preserve-missing-items'

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