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>