t5503: fix overspecification of trace expectation

In order to extract the wants from a trace, a loop in t5503 currently
breaks if "0000" is found. This works for protocol v0 and v1, but not
v2. Instead, teach t5503 to look specifically for the "want" string,
which is compatible with all protocols.

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

t/t5503-tagfollow.sh

@@ -47,7 +47,7 @@ get_needs () {
 	test -s "$1" &&
 	perl -alne '
 		next unless $F[1] eq "upload-pack<";
-		last if $F[2] eq "0000";
+		next unless $F[2] eq "want";
 		print $F[2], " ", $F[3];
 	' "$1"
 }