There are a couple of options available, you can run an open source system such as GIT or SVN for versioning management, though since most of your files will likely be binary; there are a number of disadvantages to this.
There is at least one commercial multiple vendor source management system, though I cannot remember what its called at the moment. If I remember it I'll post it up here. (I've done a half dozen searches already, its not coming up).
We'd only looked at the commercial option briefly, then went to SVN as its open source and we had a lot less issues justifying purchase cost on this basis. Any diffing or compare functions are still done via the native PLC software, as SVN diff doesn't support our binaries.
Someone elsewhere mentioned that GIT has an advantage in that every repository is a full local copy, whereas for updates SVN needs a remote server. We (attempt) to manage this by trying to ensure that releases are done before people head to site, but it doesn't always happen.
I don't have any experience with GE explicitly though, all our gear is either Omron or AB.
