shortlog: optimize out useless "<none>" normalization

shortlog: optimize out useless "<none>" normalization

If we are in --summary mode, we will always pass <none> to
insert_one_record, which will then do some normalization
(e.g., cutting out "[PATCH]"). There's no point in doing so
if we aren't going to use the result anyway.

This drops my best-of-five for "git shortlog -ns HEAD" on
linux.git from:

  real    0m5.257s
  user    0m5.104s
  sys     0m0.156s

to:

  real    0m5.194s
  user    0m5.028s
  sys     0m0.168s

That's only 1%, but arguably the result is clearer to read,
as we're able to group our variable declarations inside the
conditional block. It also opens up further optimization
possibilities for future patches.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>