commit-graph: define common usage with a macro
Share the usage message between these three variables by using a
macro. Before this new options needed to copy/paste the usage
information, see e.g.
809e0327f5 (builtin/commit-graph.c: introduce
'--max-new-filters=<n>', 2020-09-18).
See
b25b727494f (builtin/multi-pack-index.c: define common usage with
a macro, 2021-03-30) for another use of this pattern (but on-list this
one came first).
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>