Rebase may misbehave on concurrent updates of the same row

[wonder-sk]

If there is a situation that users A and B are starting from a common base database and then:

• user A modifies column X of feature 123
• user B modifies column Y of feature 123

Then if user B does rebase of their changes on top of user A's changes, geodiff would:

• create a conflict file which is not even correct (it would only contain the "old" value for a conflict, but not the "base" and "new" values)
• create a malformed rebased diff - containing "old" value for a column, but not containing "new" value