This uncovered a bug in diff_algo_none(), so let's keep it.

Now allocating patience specific data only when patience is going to be used. This may allow using forward-Myers more often, since the state size per line is significantly smaller. Patience instead allocates an array, puts it in the current diff_data, and also place a pointer of the current diff_data in the root diff_data (since each atom points to the root diff_data).

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.

-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).