diff.c: rewrite emit_line_0 more understandably

diff.c: rewrite emit_line_0 more understandably

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: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>