Add testcase for needless objects during a shallow fetch

This is a testcase that checks for a problem where, during a specific
shallow fetch where the client does not have any commits that are a
successor of the new shallow root (i.e., the fetch creates a new
detached piece of history), the server would simply send over _all_
objects, instead of taking into account the objects already present in
the client.

The actual problem was fixed by a recent patch series by Nguyễn Thái
Ngọc Duy already.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5500-fetch-pack.sh

@@ -393,6 +393,17 @@ test_expect_success 'fetch in shallow repo unreachable shallow objects' '
 		git fsck --no-dangling
 	)
 '
+test_expect_success 'fetch creating new shallow root' '
+	(
+		git clone "file://$(pwd)/." shallow10 &&
+		git commit --allow-empty -m empty &&
+		cd shallow10 &&
+		git fetch --depth=1 --progress 2>actual &&
+		# This should fetch only the empty commit, no tree or
+		# blob objects
+		grep "remote: Total 1" actual
+	)
+'
 
 test_expect_success 'setup tests for the --stdin parameter' '
 	for head in C D E F