Emit a whole line in one go

Emit a whole line in one go

Since the graph prefix will be printed when calling
emit_line, so the functions should be used to emit a
complete line out once a time. No one should call
emit_line to just output some strings instead of a
complete line.
Use a strbuf to compose the whole line, and then
call emit_line to output it once.

Signed-off-by: Bo Yang <struggleyb.nku@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>