diff: refactor the word-diff setup from builtin_diff_cmd

Quite a chunk of builtin_diff_cmd deals with word-diff setup, defaults
and such.  This makes the function a bit hard to read, but is also
asymmetric because the corresponding teardown lives in free_diff_words_data
already.

Refactor into a new function init_diff_words_data.  For simplicity,
also shuffle around some functions it depends on.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>