send-email: lazily shell out to "git var"

send-email: lazily shell out to "git var"

Optimize git-send-email by only shelling out to "git var" if we need
to. This is easily done by re-inventing our own small version of
perl's Memoize module.

I suppose I could just use Memoize itself, but in a subsequent patch
I'll be micro-optimizing send-email's use of dependencies. Using
Memoize is a measly extra 5-10 milliseconds, but as we'll see that'll
end up mattering for us in the end.

This brings the runtime of a plain "send-email" from around ~160-170ms
to ~140m-150ms. The runtime of the tests is around the same, or around
~20s.

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