t6200: use test_commit_bulk

t6200: use test_commit_bulk

There's a loop that creates 30 commits using test_commit. Using
test_commit_bulk speeds this up from:

  Benchmark #1: ./t6200-fmt-merge-msg.sh --root=/var/ram/git-tests
    Time (mean ± σ):      1.926 s ±  0.240 s    [User: 1.055 s, System: 0.963 s]
    Range (min … max):    1.431 s …  2.166 s    10 runs

to:

  Benchmark #1: ./t6200-fmt-merge-msg.sh --root=/var/ram/git-tests
    Time (mean ± σ):      1.343 s ±  0.179 s    [User: 766.5 ms, System: 662.9 ms]
    Range (min … max):    1.032 s …  1.664 s    10 runs

for an average savings of over 30%.

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