t/test-lib-functions.sh: support `--notick` in `test_commit_bulk()`

t/test-lib-functions.sh: support `--notick` in `test_commit_bulk()`

One of the tests we'll want to add for pseudo-merge bitmaps needs to be
able to generate a large number of commits at a specific date.

Support the `--notick` option (with identical semantics to the
`--notick` option for `test_commit()`) within `test_commit_bulk` as a
prerequisite for that. Callers can then set the various _DATE variables
themselves.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>