t5500: give fully-qualified refs to fetch-pack

The fetch-pack documentation is very clear that refs given
on the command line are to be full refs:

  <refs>...::
          The remote heads to update from. This is relative to
          $GIT_DIR (e.g. "HEAD", "refs/heads/master").  When
          unspecified, update from all heads the remote side has.

and this has been the case since fetch-pack was originally documented in
8b3d9dc ([PATCH] Documentation: clone/fetch/upload., 2005-07-14).

Let's follow our own documentation to set a good example,
and to avoid breaking when this restriction is enforced in
the next patch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

t/t5500-fetch-pack.sh

@@ -97,7 +97,7 @@ test_expect_success 'setup' '
 	git symbolic-ref HEAD refs/heads/B
 '
 
-pull_to_client 1st "B A" $((11*3))
+pull_to_client 1st "refs/heads/B refs/heads/A" $((11*3))
 
 test_expect_success 'post 1st pull setup' '
 	add A11 $A10 &&
@@ -110,9 +110,9 @@ test_expect_success 'post 1st pull setup' '
 	done
 '
 
-pull_to_client 2nd "B" $((64*3))
+pull_to_client 2nd "refs/heads/B" $((64*3))
 
-pull_to_client 3rd "A" $((1*3))
+pull_to_client 3rd "refs/heads/A" $((1*3))
 
 test_expect_success 'clone shallow' '
 	git clone --depth 2 "file://$(pwd)/." shallow