t5539: update a flaky test

The test creates some unrelated commits in two separate repositories,
and then fetches from one to the other. Since the commit creation
happens in a subshell, the first commit in each ends up with the
same test_tick value. When fetch-pack looks at the two root commits
"unrelated1" and "new-too", the exact sequence of ACKs is different
depending on which one it pulls out of the queue first.

With the current code, it happens to be "unrelated1" (though this is not
at all guaranteed by the prio_queue data structure, it is deterministic
for this particular sequence of input). We see the ready-ACK, and the
test succeeds.

With the stable queue, we reliably get "new-too" out (since it is our
local tip, it is added to the queue before we even talk to the remote).
We never see a ready-ACK, and the test fails due to the grep on the
TRACE_PACKET output at the end (the fetch itself succeeds as expected).

I'm really not quite clear on what's supposed to be going on in the
test. I can make it pass with this change.

t/t5539-fetch-http-shallow.sh

@@ -54,6 +54,7 @@ EOF
 test_expect_success 'no shallow lines after receiving ACK ready' '
 	(
 		cd shallow &&
+		test_tick &&
 		for i in $(test_seq 15)
 		do
 			git checkout --orphan unrelated$i &&