Larry Wall's patch was a huge step forward. In case you don't know what this is, it takes the output of diff and tries to apply those diffs to your source file

Larry Wall's patch was a huge step forward. In case you don't know what this is, it takes the output of diff and tries to apply those diffs to your source files. If diffed baseline and your baseline are the same then the patch will apply cleanly. If they are close the patch may apply cleanly (though it may have some messages like "offset 3 lines" which tells you that it found the place to patch 3 lines away from where it thought it was). There was a big period of time where the Linux kernel work was all diff/patch based. So the answer was (a) a mailing list that everyone was one, (b) some way of knowing a baseline (you could just have a baseline tree and a copy of that and you diff -Nur the two trees), (c) email the patch. All that worked but it didn't scale which is why Linus switched to BitKeeper and then later Git.