git transmat should cache checkout tree of submodules

[warpfork]

The git transmat currently caches the git objects (in their internal content-addressable layouts), and caches full checkouts of the final fileset (including all submodules). So far so good.

It should also cache the fileset of the submodule checkouts. The time it takes to do a git checkout from the object cache is quite large compared to how long it takes to do a simple copy of the files if they're already on disk.

This would increase the amount of disk space used in caching, but increase speed significantly in the case of repos with many submodules where the submodules don't typically change between commits of the main parent repo (which is basically normal usage).