diff.c: shuffling code around

diff.c: shuffling code around

Move function, type, and structure definitions for fill_mmfile(),
count_trailing_blank(), check_blank_at_eof(), emit_line(),
new_blank_line_at_eof(), emit_add_line(), sane_truncate_fn, and
emit_callback up in the file, so that they can be refactored into helper
functions and reused by codepath for emitting rewrite patches.

This only moves the lines around to make the next two patches easier to
read.

Signed-off-by: Junio C Hamano <gitster@pobox.com>