There is actually no good way to accomplish this using SVN. The best thing you can do is to warn users to update in a certain way for the next update by copying the file and then back after the update.
Sometimes the easiest (or only) way to solve technical problems is through "social engineering".
Finally, I would suggest you give up SVN when you have a chance. Other options out there, such as Mercurial or Git, are much more forgiving.
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