log: avoid loading decorations for userformats that don't need it

If no --decorate option is given, we default to auto-decoration. And
when that kicks in, cmd_log_init_finish() will unconditionally load the
decoration refs.

However, if we are using a user-format that does not include "%d" or
"%D", we won't show the decorations at all, so we don't need to load
them. We can detect this case and auto-disable them by adding a new
field to our userformat_want helper. We can do this even when the user
explicitly asked for --decorate, because it can't affect the output at
all.

This patch consistently reduces the time to run "git log -1 --format=%H"
on my git.git clone (with ~2k refs) from 34ms to 7ms. On a much more
extreme real-world repository (with ~220k refs), it goes from 2.5s to
4ms.

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

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