Note: This is all from memory, but I believe the commands are correct. Download a merge program called Meld. Assuming you have a remote called upstream which points to OpenJK (forking on Github should do this, but I can't remember) Open a git prompt and type: git fetch upstream
git rebase upstream/masterAssuming there are merge conflicts (i.e. you've modified the same line of code as someone else working on OpenJK) you will have to do a 2-way merge.Meld will open with 3 views. I can't quite remember which view is what, but it should be fairly obvious. Remember each merge will have rewound history to the point in time each commit happened. You want to merge your final changes into the middle file, and hit save. It may take you a few tries to get this working, but don't worry if you screw up a merge, you can do a hard-reset of history via: git reflog # to see which "ref" or "state" to revert to
git reset --hard HEAD@{1}