upload-pack: teach deepen-relative in protocol v2

Commit 685fbd3291 ("fetch-pack: perform a fetch using v2", 2018-03-15)
attempted to teach Git deepen-relative in protocol v2 (among other
things), but it didn't work:

 (1) fetch-pack.c needs to emit "deepen-relative".
 (2) upload-pack.c needs to ensure that the correct deepen_relative
     variable is passed to deepen() (there are two - the static variable
     and the one in struct upload_pack_data).
 (3) Before deepen() computes the list of reachable shallows, it first
     needs to mark all "our" refs as OUR_REF. v2 currently does not do
     this, because unlike v0, it is not needed otherwise.

Fix all this and include a test demonstrating that it works now. For
(2), the static variable deepen_relative is also eliminated, removing a
source of confusion.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Reviewed-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5702-protocol-v2.sh

@@ -489,6 +489,35 @@ test_expect_success 'ensure that multiple fetches in same process from a shallow
 	grep "fetch< version 2" trace
 '
 
+test_expect_success 'deepen-relative' '
+	rm -rf server client trace &&
+
+	test_create_repo server &&
+	test_commit -C server one &&
+	test_commit -C server two &&
+	test_commit -C server three &&
+	git clone --depth 1 "file://$(pwd)/server" client &&
+	test_commit -C server four &&
+
+	# Sanity check that only "three" is downloaded
+	git -C client log --pretty=tformat:%s master >actual &&
+	echo three >expected &&
+	test_cmp expected actual &&
+
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client -c protocol.version=2 \
+		fetch --deepen=1 origin &&
+	# Ensure that protocol v2 is used
+	grep "fetch< version 2" trace &&
+
+	git -C client log --pretty=tformat:%s origin/master >actual &&
+	cat >expected <<-\EOF &&
+	four
+	three
+	two
+	EOF
+	test_cmp expected actual
+'
+
 # Test protocol v2 with 'http://' transport
 #
 . "$TEST_DIRECTORY"/lib-httpd.sh