How do I "merge" two text files?

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http://unix.stackexchange.com – I recently upgraded from Ubuntu 12.04 to 12.10 and at one point, it encountered an Apache config file conflict in apache2.conf. I didn't give me a merge option at that point, so I just rejected the new file and the installer saved the new file as apache2.conf.dpkg-dist. I can diff the two files with diff apache2.conf apache2.conf.dpkg-dist and get just the lines that are different. But I want to manually merge the two sort of like how I resolve merge conflicts in SVN or git. How can I do that? (HowTos)