Isolate git index from graph

[kofalt]

As per @heavenlyhash, it is allegedly possible to isolate the index (staging area) from the graph with some git flags. We should map this to a temporary folder in the same manner we map working copies to further isolate unwanted graph state.

Flag is GIT_INDEX_FILE, as per the man page:

This environment allows the specification of an alternate index file. If not specified, the default of $GIT_DIR/index is used.