Commit Briefs
rename diff_atom->d to diff_atom->root, because it always is
The idea was that for each diff box within the files, the atoms would have a backpointer to the current layer of diff_data (indicating the current section), but it is not actually needed to update the backpointer in each atom to the current diff_data. That is why the current code always points atom->d to the root diff_data for the entire file. Clarify by proper name. Constructs like atom->d->root->foo are redundant, just use atom->root->foo.
diff: drop -p, add -P -Q -T; diffreg(): use algo enum
-p is traditionally used for "show which C function each change is in", so don't use it for choosing Patience. Instead add two options for using Patience, -P for only Patience and -Q to attempt forward-Myers first. Also add -T for a trivial diff (diff_algo_none).