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 b25b727494 (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>

builtin/commit-graph.c

@@ -9,26 +9,29 @@
 #include "progress.h"
 #include "tag.h"
 
-static char const * const builtin_commit_graph_usage[] = {
-	N_("git commit-graph verify [--object-dir <objdir>] [--shallow] [--[no-]progress]"),
-	N_("git commit-graph write [--object-dir <objdir>] [--append] "
-	   "[--split[=<strategy>]] [--reachable|--stdin-packs|--stdin-commits] "
-	   "[--changed-paths] [--[no-]max-new-filters <n>] [--[no-]progress] "
-	   "<split options>"),
+#define BUILTIN_COMMIT_GRAPH_VERIFY_USAGE \
+	N_("git commit-graph verify [--object-dir <objdir>] [--shallow] [--[no-]progress]")
+
+#define BUILTIN_COMMIT_GRAPH_WRITE_USAGE \
+	N_("git commit-graph write [--object-dir <objdir>] [--append] " \
+	   "[--split[=<strategy>]] [--reachable|--stdin-packs|--stdin-commits] " \
+	   "[--changed-paths] [--[no-]max-new-filters <n>] [--[no-]progress] " \
+	   "<split options>")
+
+static const char * builtin_commit_graph_verify_usage[] = {
+	BUILTIN_COMMIT_GRAPH_VERIFY_USAGE,
 	NULL
 };
 
-static const char * const builtin_commit_graph_verify_usage[] = {
-	N_("git commit-graph verify [--object-dir <objdir>] [--shallow] [--[no-]progress]"),
+static const char * builtin_commit_graph_write_usage[] = {
+	BUILTIN_COMMIT_GRAPH_WRITE_USAGE,
 	NULL
 };
 
-static const char * const builtin_commit_graph_write_usage[] = {
-	N_("git commit-graph write [--object-dir <objdir>] [--append] "
-	   "[--split[=<strategy>]] [--reachable|--stdin-packs|--stdin-commits] "
-	   "[--changed-paths] [--[no-]max-new-filters <n>] [--[no-]progress] "
-	   "<split options>"),
-	NULL
+static char const * const builtin_commit_graph_usage[] = {
+	BUILTIN_COMMIT_GRAPH_VERIFY_USAGE,
+	BUILTIN_COMMIT_GRAPH_WRITE_USAGE,
+	NULL,
 };
 
 static struct opts_commit_graph {