Well, one available case you can look at is Uru: Live / Myst Online, currently running under the name Myst Online: Uru Live: Again.
They open-sourced their Dirt/Headspin/Plasma engine, which required stripping out - among other things - the PhysX code from it.
Mercurial does have a few things going for it, though for most use-cases it's behind Git in almost all metrics.
I really do like the fact that it keeps a commit number counter, it's a lot easier to know if "commit 405572" is newer than "commit 405488" after all, instead of Git's "commit ea43f56" vs "commit ab446f1". (Though Git does have the describe format, which helps somewhat in this regard. E.g. "0.95b-4204-g1e97859fb" being the 4204th commit after tag 0.95b)