Barebones 2-way diff thread. Further development should include:
- recognize at least %txt and %hoon diff types as (urge:clay cord) and
print them prettily
- check if they're directories and recurse through them, noting files
that exist in one or the other and diffing ones that exist in both
- support 3-way diffs. Since we know the desks they're on, we can find
a mergebase (see how +trouble gets the base-hash in #3378) and do a
3-way diff.
- extend the above to support all the useful diffs, including
diff3(a,b), diff2(mergebase(a,b),a) and any other useful ones.
