We'd like to see a complete source tree at once

Two reasons:
Most changes modify several files at once, but current systems only let you look at the changes to one file at a time.
Rearranging the directory structure is itself a change which should be visible to later archaeologists.
File-level granularity is fine
We can always zoom in on the changes to each individual file. Being able to get context diffs for the whole change would be useful, though.
What should it look like?
Several possibilities:
Most version control systems don't record enough information
A change that modifies several files is indistinguishable from a change to several files at once. Also, many systems can't handle rearranging files at all. This is a known problem; the next generation of VC systems will give a tree browser all the information it needs.

That's all, folks!