send-email: lazily load modules for a big speedup

send-email: lazily load modules for a big speedup

Optimize the time git-send-email takes to do even the simplest of
things (such as serving up "-h") from around ~150ms to ~80ms-~90ms by
lazily loading the modules it requires.

Before this change Devel::TraceUse would report 99/97 used modules
under NO_GETTEXT=[|Y], respectively. Now it's 52/37. It now takes ~15s
to run t9001-send-email.sh, down from ~20s.

Changing File::Spec::Functions::{catdir,catfile} to invoking class
methods on File::Spec itself is idiomatic. See [1] for a more
elaborate explanation, the resulting code behaves the same way, just
without the now-pointless function wrapper.

1. http://lore.kernel.org/git/8735u8mmj9.fsf@evledraar.gmail.com

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>