tests: don't give unportable ">" to "test" built-in, use -gt

tests: don't give unportable ">" to "test" built-in, use -gt

Change an argument to test_line_count (which'll ultimately be turned
into a "test" expression) to use "-gt" instead of ">" for an
arithmetic test.

This broken on e.g. OpenBSD as of v2.13.0 with my commit
ac3f5a3468 ("ref-filter: add --no-contains option to
tag/branch/for-each-ref", 2017-03-24).

Downstream just worked around it by patching git and didn't tell us
about it, I discovered this when reading various Git packaging
implementations: https://github.com/openbsd/ports/commit/7e48bf88a20

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>