First, you shouldn't include generated or 'compiled' files in you version control repository. See this SO answer for the reasoning about it. The best option would be to have a build script that generates the combined files with a simple command and add it to your deploy process. The generated files should be added to the svn:ignore property.
For your specific problem, there are even web server modules, like Google mod_pagespeed, that automatically combine javascript files. This solution is a lot better than including generated code in your repository.
Now, if you really want to do it, you should do with a pre-commit hook. You should generate your files before sending them to version control. See that you will have a lot of trouble with it:
- First: the subversion hooks run in the machine where is your subversion server, so you need all the scripts that will generate the files located there.
- The consequence is that you won't have the generated files in your local workspace, you'll need to do an update after each commit. Terrible!
- You have to take care to add the file to subversion if it isn't under version control
- If for any reason there is a conflict commiting this file, your users will have a hell of a time trying to debug it.
If you follow this monstruous path, please don't tell anyone that I've told you how to do it :-)