i18n: mark checkout plural warning for translation

Mark the "Warning: you are leaving %d commit(s) behind" message added
in v1.7.5-rc0~74^2 (commit: give final warning when reattaching HEAD
to leave commits behind) by Junio C Hamano for translation.

This message requires the use of ngettext() features, and is the first
message to use the Q_() wrapper around ngettext().

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -648,18 +648,30 @@ static void suggest_reattach(struct commit *commit, struct rev_info *revs)
 		if (more == 1)
 			describe_one_orphan(&sb, last);
 		else
-			strbuf_addf(&sb, " ... and %d more.\n", more);
+			strbuf_addf(&sb, _(" ... and %d more.\n"), more);
 	}
 
 	fprintf(stderr,
-		"Warning: you are leaving %d commit%s behind, "
+		Q_(
+		/* The singular version */
+		"Warning: you are leaving %d commit behind, "
+		"not connected to\n"
+		"any of your branches:\n\n"
+		"%s\n"
+		"If you want to keep it by creating a new branch, "
+		"this may be a good time\nto do so with:\n\n"
+		" git branch new_branch_name %s\n\n",
+		/* The plural version */
+		"Warning: you are leaving %d commits behind, "
 		"not connected to\n"
 		"any of your branches:\n\n"
 		"%s\n"
 		"If you want to keep them by creating a new branch, "
 		"this may be a good time\nto do so with:\n\n"
 		" git branch new_branch_name %s\n\n",
-		lost, ((1 < lost) ? "s" : ""),
+		/* Give ngettext() the count */
+		lost),
+		lost,
 		sb.buf,
 		sha1_to_hex(commit->object.sha1));
 	strbuf_release(&sb);

t/t2020-checkout-detach.sh

@@ -108,21 +108,30 @@ test_expect_success 'checkout warns on orphan commits' '
 	echo content >orphan &&
 	git add orphan &&
 	git commit -a -m orphan &&
-	git checkout master 2>stderr &&
+	git checkout master 2>stderr
+'
+
+test_expect_success C_LOCALE_OUTPUT 'checkout warns on orphan commits: output' '
 	check_orphan_warning stderr
 '
 
 test_expect_success 'checkout does not warn leaving ref tip' '
 	reset &&
 	git checkout --detach two &&
-	git checkout master 2>stderr &&
+	git checkout master 2>stderr
+'
+
+test_expect_success C_LOCALE_OUTPUT 'checkout does not warn leaving ref tip' '
 	check_no_orphan_warning stderr
 '
 
 test_expect_success 'checkout does not warn leaving reachable commit' '
 	reset &&
 	git checkout --detach HEAD^ &&
-	git checkout master 2>stderr &&
+	git checkout master 2>stderr
+'
+
+test_expect_success C_LOCALE_OUTPUT 'checkout does not warn leaving reachable commit' '
 	check_no_orphan_warning stderr
 '