The commit history does not accurately reflect the contributions to the code-base. As I understand it, contributors would write and provide patches to Bram who would then commit the changes to the code-base.
Why would anyone amend the author in an open source project?
From what I can tell the reason is that that’s the way it was always done. As I understand it, back in the days before there were distributed version control systems, Bram was the only one who could commit to the code-base so he would credit the actual author of a submitted patch in the commit message. He continued this practice after the Vim project moved to Mercurial (and Git).
Related discussion: https://github.com/vim/vim/issues/1554
Ah, thanks
On reflection, it might have been clearer to have started my comment with:
“Taken at face value by most modern users of Git, the commit history does not accurately reflect the contributions to the code-base”.
Looking at the commit history, vim was largely a one-man show, I'm a bit skeptical about that changing basically overnight.