1. collect a the minus/plus lines of a diff hunk, divided into
minus-lines and plus-lines;
2. break both minus-lines and plus-lines into words and
place them into two mmfile_t with one word for each line;
3. use xdiff to run diff on the two mmfile_t to get the words level diff;
And for the common parts of the both file, we output the plus side text.
diff_words->current_plus is used to trace the current position of the plus file
which printed. diff_words->last_minus is used to trace the last minus word
printed.
For '--graph' to work with '--color-words', we need to output the graph prefix
on each line of color words output. Generally, there are two conditions on
which we should output the prefix.