Add an expensive test for git-notes

Add an expensive test for git-notes

git-notes have the potential of being pretty expensive, so test with
a lot of commits.  A lot.  So to make things cheaper, you have to
opt-in explicitely, by setting the environment variable
GIT_NOTES_TIMING_TESTS.

This patch has been improved by the following contributions:
- Junio C Hamano: tests: fix "export var=val"

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>