help tests: test "git" and "git help [-a|-g] spacing

help tests: test "git" and "git help [-a|-g] spacing

There's logic in "help.c"'s "print_cmd_by_category()" to emit "help"
output with particular spacing, which doesn't make much sense when
emitting only one section with "help -g".

Let's add tests for the current spacing in preparation for a
subsequent whitespace formatting fix, and make sure that that fix
doesn't cause regressions for the "git" and "git help" output.

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