log --use-mailmap: optimize for cases without --author/--committer search

When we taught the commit_match() mechanism to pay attention to the
new --use-mailmap option, we started to unconditionally copy the
commit object to a temporary buffer, just in case we need the author
and committer lines updated via the mailmap mechanism, and rewrite
author and committer using the mailmap.

It turns out that this has a rather unpleasant performance
implications.  In the linux kernel repository, running

  $ git log --author='Junio C Hamano' --pretty=short >/dev/null

under /usr/bin/time, with and without --use-mailmap (the .mailmap
file is 118 entries long, the particular author does not appear in
it), cost (with warm cache):

  [without --use-mailmap]
  5.42user 0.26system 0:05.70elapsed 99%CPU (0avgtext+0avgdata 2005936maxresident)k
  0inputs+0outputs (0major+137669minor)pagefaults 0swaps

  [with --use-mailmap]
  6.47user 0.30system 0:06.78elapsed 99%CPU (0avgtext+0avgdata 2006288maxresident)k
  0inputs+0outputs (0major+137692minor)pagefaults 0swaps

which incurs about 20% overhead.  The command is doing extra work,
so the extra cost may be justified.

But it is inexcusable to pay the cost when we do not need
author/committer match.  In the same repository,

  $ git log --grep='fix menuconfig on debian lenny' --pretty=short >/dev/null

shows very similar numbers as the above:

  [without --use-mailmap]
  5.32user 0.30system 0:05.63elapsed 99%CPU (0avgtext+0avgdata 2005984maxresident)k
  0inputs+0outputs (0major+137672minor)pagefaults 0swaps

  [with --use-mailmap]
  6.64user 0.24system 0:06.89elapsed 99%CPU (0avgtext+0avgdata 2006320maxresident)k
  0inputs+0outputs (0major+137694minor)pagefaults 0swaps

The latter case is an unnecessary performance regression.  We may
want to _show_ the result with mailmap applied, but we do not have
to copy and rewrite the author/committer of all commits we try to
match if we do not query for these fields.

Trivially optimize this performace regression by limiting the
rewrites for only when we are matching with author/committer fields.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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