Make usage strings dash-less

When you misuse a git command, you are shown the usage string.
But this is currently shown in the dashed form.  So if you just
copy what you see, it will not work, when the dashed form
is no longer supported.

This patch makes git commands show the dash-less version.

For shell scripts that do not specify OPTIONS_SPEC, git-sh-setup.sh
generates a dash-less usage string now.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-for-each-ref.c

@@ -831,7 +831,7 @@ int opt_parse_sort(const struct option *opt, const char *arg, int unset)
 }
 
 static char const * const for_each_ref_usage[] = {
-	"git-for-each-ref [options] [<pattern>]",
+	"git for-each-ref [options] [<pattern>]",
 	NULL
 };