Comment # 3 on bug 55759 from Patrick Ohly

The design decision is that the engine stores a vCard that is not the one that
it received. Instead, conceptually, it stores the result of merging local and
remote data, which may lead to a different result and thus requires bumping the
REV string.

The "cache data in local file" is a special case where no merging happens, but
it still follows the same rules in the engine. Changing that requires a
different approach.