I want to find a tool so that I can diff two files A and B, and apply those changes to a third file C, to create a fourth file D.
kdiff3 is pretty close to what I want, but I've had some issues with its diff algorithm. DiffMerge is also pretty close, but seems to be more for situations where there is a reasonably recent common ancestor that the changes are applied to. This is not the case for the situation I am normally dealing with. I'm not interested in the changes between A and C or B and C. P4Merge doesn't handle the file sizes that I'm using.
Ideally I'd like to confirm each change done to C to create D, but just at the click of a button or something. This seems like it should be a reasonably common problem. I'm not really after a command-line does-it-all-in-one-go app, because I would like to know what changes are being applied - I don't have that trust level yet, although maybe that could be solved with a post diff.
I don't now much about the whole unified diff thing, but assume that it may not be suitable, because A and B are usually not ancestors of C, although they do have a common ancestor.