serve: introduce the server-option capability

Introduce the "server-option" capability to protocol version 2.  This
enables future clients the ability to send server specific options in
command requests when using protocol version 2.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5701-git-serve.sh

@@ -10,6 +10,7 @@ test_expect_success 'test capability advertisement' '
 	agent=git/$(git version | cut -d" " -f3)
 	ls-refs
 	fetch=shallow
+	server-option
 	0000
 	EOF
 
@@ -173,4 +174,24 @@ test_expect_success 'symrefs parameter' '
 	test_cmp actual expect
 '
 
+test_expect_success 'sending server-options' '
+	test-pkt-line pack >in <<-EOF &&
+	command=ls-refs
+	server-option=hello
+	server-option=world
+	0001
+	ref-prefix HEAD
+	0000
+	EOF
+
+	cat >expect <<-EOF &&
+	$(git rev-parse HEAD) HEAD
+	0000
+	EOF
+
+	git serve --stateless-rpc <in >out &&
+	test-pkt-line unpack <out >actual &&
+	test_cmp actual expect
+'
+
 test_done