p5302: create repositories for index-pack results explicitly

p5302: create repositories for index-pack results explicitly

Before 7176a314 (index-pack: complain when --stdin is used outside of a
repo) index-pack silently created a non-existing target directory; now
the command refuses to work unless it's used against a valid repository.
That causes p5302 to fail, which relies on the former behavior.  Fix it
by setting up the destinations for its performance tests using git init.

Signed-off-by: Rene Scharfe <redacted>
Acked-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>