This repository uses subtrees to manage some of its externals.
Initial setup
git remote add externals-fmt https://github.com/fmtlib/fmt.git
git remote add externals-mp https://github.com/MerryMage/mp.git
git remote add externals-xbyak https://github.com/herumi/xbyak.git
Updating
Change <ref> to refer to the appropriate git reference.
git fetch externals-fmt
git fetch externals-mp
git fetch externals-xbyak
git subtree pull --squash --prefix=externals/fmt externals-fmt <ref>
git subtree pull --squash --prefix=externals/mp externals-mp <ref>
git subtree pull --squash --prefix=externals/xbyak externals-xbyak <ref>