t64xx: convert 'test_create_repo' to 'git init'

t64xx: convert 'test_create_repo' to 'git init'

Convert the merge-specific tests (those in the t64xx range) over to
using 'git init' instead of 'test_create_repo'.

Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>