t5703: use test_commit_bulk

t5703: use test_commit_bulk

There are two loops that create 33 commits each using test_commit. Using
test_commit_bulk speeds this up from:

  Benchmark #1: ./t5703-upload-pack-ref-in-want.sh --root=/var/ram/git-tests
    Time (mean ± σ):      2.142 s ±  0.161 s    [User: 1.136 s, System: 0.974 s]
    Range (min … max):    1.903 s …  2.401 s    10 runs

to:

  Benchmark #1: ./t5703-upload-pack-ref-in-want.sh --root=/var/ram/git-tests
    Time (mean ± σ):      1.440 s ±  0.114 s    [User: 737.7 ms, System: 615.4 ms]
    Range (min … max):    1.230 s …  1.604 s    10 runs

for an average savings of almost 33%.

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