move patience data out of struct diff_atom 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).

patience: use qsort to optimize finding unique lines

cosmetic: indents in diff_main.h

move headers from include/diff/ to include/; makes things easier for consumers