reverter plugin: ignores any changes that come after the change set that is to be reverted

[bastiK]

• changeset 1: add a POI
• changeset 2: add note="I was there"
• changeset 3: add addr:housenumber=23
• revert the spam cs 2
• result: the useful information from cs 3 is gone without warning
• expected result: create a conflict. (Treat current version as if it was 'modified'.)

[Upliner]

OK. Now if revert was "clean", primitive version won't be set back. But if revert was 'dirty', primitive version will be set back and when you'll try to upload this data you'll get a conflict.

[bastiK]

Nice solution. On the long run it might be better to create these conflicts right away (see #5004). After all, we already know if the revert is "dirty", so in case it is "clean" there is no more network traffic.