t3302-notes-index-expensive: Speed up create_repo()

t3302-notes-index-expensive: Speed up create_repo()

Creating repos with 10/100/1000/10000 commits and notes takes a lot of time.
However, using git-fast-import to do the job is a lot more efficient than
using plumbing commands to do the same.

This patch decreases the overall run-time of this test on my machine from
~3 to ~1 minutes.

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