diff.c: split emit_line() from the first char and the rest of the line

diff.c: split emit_line() from the first char and the rest of the line

A new helper function emit_line_0() takes the first line of diff output
(typically "-", " ", or "+") separately from the remainder of the line.
No other functional changes.

This change will make it easier to reuse the logic when emitting the
rewrite diff, as we do not want to copy a line only to add "+"/"-"/" "
immediately before its first character when we produce rewrite diff
output.

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