Rewrite emit_line_0 to have fewer (nested) conditions.

The change in 'emit_line' makes sure that 'first' is never user data,
but always under our control, a sign or special character in the
beginning of the line (or 0, in which case we ignore it).
So from now on, let's pass only a diff marker or 0 as the 'first'
character of the line.
Signed-off-by: NStefan Beller <sbeller@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>