switch: add short option for --detach

"git checkout" automatically detaches branches and --detach is not
that useful (--no-detach is more likely). But for "switch", you
may want to use it more often once you're used to detached HEAD. This
of course adds -d to git-checkout but it does not harm (yet?) to do it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -1423,7 +1423,7 @@ static struct option *add_common_switch_branch_options(
 	struct checkout_opts *opts, struct option *prevopts)
 {
 	struct option options[] = {
-		OPT_BOOL(0, "detach", &opts->force_detach, N_("detach HEAD at named commit")),
+		OPT_BOOL('d', "detach", &opts->force_detach, N_("detach HEAD at named commit")),
 		OPT_SET_INT('t', "track",  &opts->track, N_("set upstream info for new branch"),
 			BRANCH_TRACK_EXPLICIT),
 		OPT_STRING(0, "orphan", &opts->new_orphan_branch, N_("new-branch"), N_("new unparented branch")),