I think this fixes the Mercurial + HTTP cluster issue. PHP adds HTTP_ but we were not stripping it, so we would convert an X-Whatever-Zebra header into an Http-X-Whatever-Zebra header.
I don't think this behavior has changed? So maybe it just never worked? Git is more popular than Mercurial and SSH is easier to configure than HTTP, so it's plausible. I'll keep a careful eye on this when it deploys.