Git overhaul -- fetch/checkout/submodule splits

[warpfork]

Massive overhaul to the git transmat, which should increase efficiency and successful caching.

• Submodules are now cached: if you have a parent project move to a new commit, but keep all the same submodule versions, previously repeatr was oblivious; now, it's a fast 100% cache hit.
• Git data objects are now cached: fetches are incremental when using the same remote repos.
• Slight change: ".git" files in submodule paths no longer leak into the container's sight. This shouldn't affect you unless you had output configurations exporting filesystems including such files.

[warpfork]

Whee, re-checked repeat-thyself.sh with a build of repeatr before and after this branch -- mission accomplished! still consistent! This whole change made with no external impacts.