HOME must be set before calling git-init when creating test repositories

Otherwise the created test repositories will be affected by users ~/.gitconfig.
For example, setting core.logAllrefupdates in users config will make all
calls to "git config --unset core.logAllrefupdates" fail which will break
the first test which uses the statement and expects it to succeed.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>