git transmat should parallelize more (and network less)

[warpfork]

The git transmat should do a lot more work in parallel.

There's probably some very low-hanging fruit here.

Particularly, for submodules to be fetched in parallel once the main repo checkout is available should be possible. (It should even be possible in parallel with the actual checkout -- the submodules can be read from the git data, then submodule fetches launched, before/in-parallel to the actual file content checkouts.)

Sidequest note: I have a sneaking suspicion there's also a bug that causes the git warehouse "open" functions to dial more than necessary (particularly, when the desired hash is already available in the repo)... and this may (or may not, idk) show up on the path and be easy to fix while handling the other parallelization.