checkout: do not mention detach advice for explicit --detach option

When a user asked for a detached HEAD specifically with `--detach`,
we do not need to give advice on what a detached HEAD state entails as
we can assume they know what they're getting into as they asked for it.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -655,7 +655,8 @@ static void update_refs_for_switch(const struct checkout_opts *opts,
 		update_ref(msg.buf, "HEAD", new->commit->object.oid.hash, NULL,
 			   REF_NODEREF, UPDATE_REFS_DIE_ON_ERR);
 		if (!opts->quiet) {
-			if (old->path && advice_detached_head)
+			if (old->path &&
+			    advice_detached_head && !opts->force_detach)
 				detach_advice(new->name);
 			describe_detached_head(_("HEAD is now at"), new->commit);
 		}

t/t2020-checkout-detach.sh

@@ -163,4 +163,27 @@ test_expect_success 'tracking count is accurate after orphan check' '
 	test_i18ncmp expect stdout
 '
 
+test_expect_success 'no advice given for explicit detached head state' '
+	# baseline
+	test_config advice.detachedHead true &&
+	git checkout child && git checkout HEAD^0 >expect.advice 2>&1 &&
+	test_config advice.detachedHead false &&
+	git checkout child && git checkout HEAD^0 >expect.no-advice 2>&1 &&
+	test_unconfig advice.detachedHead &&
+	# without configuration, the advice.* variables default to true
+	git checkout child && git checkout HEAD^0 >actual 2>&1 &&
+	test_cmp expect.advice actual &&
+
+	# with explicit --detach
+	# no configuration
+	test_unconfig advice.detachedHead &&
+	git checkout child && git checkout --detach HEAD^0 >actual 2>&1 &&
+	test_cmp expect.no-advice actual &&
+
+	# explicitly decline advice
+	test_config advice.detachedHead false &&
+	git checkout child && git checkout --detach HEAD^0 >actual 2>&1 &&
+	test_cmp expect.no-advice actual
+'
+
 test_done