[_ph_]

How about "infrastructure" and "interface"?

[zufallsheld]

Infrastructure is certainly not unambiguous here.

[_ph_]

Less than "porcellain", which doesn't have an obvious meaning in this context. Other terms could be "implementation" or "internal".

[rswail]

From the primary git manpage:

      GIT COMMANDS
         We divide Git into high level ("porcelain") commands and low level
         ("plumbing") commands.