builtin/commit-graph.c: remove subcommand-less usage string

The first line in 'git commit-graph's usage string indicates that this
command can be invoked without specifying a subcommand.  However, this
is not the case:

  $ git commit-graph
  usage: git commit-graph [--object-dir <objdir>]
     or: git commit-graph read [--object-dir <objdir>]
  [...]
  $ echo $?
  129

Remove this line from the usage string.

The synopsis in the manpage doesn't contain this line.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit-graph.c

@@ -8,7 +8,6 @@
 #include "object-store.h"
 
 static char const * const builtin_commit_graph_usage[] = {
-	N_("git commit-graph [--object-dir <objdir>]"),
 	N_("git commit-graph read [--object-dir <objdir>]"),
 	N_("git commit-graph verify [--object-dir <objdir>] [--shallow]"),
 	N_("git commit-graph write [--object-dir <objdir>] [--append|--split] [--reachable|--stdin-packs|--stdin-commits] <split options>"),