t5702: use test_commit_bulk

t5702: use test_commit_bulk

There are two loops that create 32 commits each using test_commit. Using
test_commit_bulk speeds this up from:

  Benchmark #1: ./t5702-protocol-v2.sh --root=/var/ram/git-tests
    Time (mean ± σ):      5.409 s ±  0.513 s    [User: 2.382 s, System: 2.466 s]
    Range (min … max):    4.633 s …  5.927 s    10 runs

to:

  Benchmark #1: ./t5702-protocol-v2.sh --root=/var/ram/git-tests
    Time (mean ± σ):      3.956 s ±  0.242 s    [User: 1.775 s, System: 1.627 s]
    Range (min … max):    3.449 s …  4.239 s    10 runs

for an average savings of over 25%.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>