t5510: prepare test refs more straightforwardly

"git fetch" was being used with contrived refspecs to create tags and
remote-tracking branches in test repositories in preparation for the
actual tests.  This is obscure and also makes one wonder whether this
is indeed just preparation or whether some side-effect of "git fetch"
is being tested.

So use the more straightforward commands "git tag" / "git update-ref"
when preparing branches in test repositories.
Signed-off-by: NMichael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t5510-fetch.sh

@@ -88,7 +88,7 @@ test_expect_success 'fetch --prune on its own works as expected' '
 	cd "$D" &&
 	git clone . prune &&
 	cd prune &&
-	git fetch origin refs/heads/master:refs/remotes/origin/extrabranch &&
+	git update-ref refs/remotes/origin/extrabranch master &&
 
 	git fetch --prune origin &&
 	test_must_fail git rev-parse origin/extrabranch
@@ -98,7 +98,7 @@ test_expect_success 'fetch --prune with a branch name keeps branches' '
 	cd "$D" &&
 	git clone . prune-branch &&
 	cd prune-branch &&
-	git fetch origin refs/heads/master:refs/remotes/origin/extrabranch &&
+	git update-ref refs/remotes/origin/extrabranch master &&
 
 	git fetch --prune origin master &&
 	git rev-parse origin/extrabranch
@@ -117,7 +117,7 @@ test_expect_success 'fetch --prune --tags does not delete the remote-tracking br
 	cd "$D" &&
 	git clone . prune-tags &&
 	cd prune-tags &&
-	git fetch origin refs/heads/master:refs/tags/sometag &&
+	git tag sometag master &&
 
 	git fetch --prune --tags origin &&
 	git rev-parse origin/master &&
@@ -128,7 +128,7 @@ test_expect_success 'fetch --prune --tags with branch does not delete other remo
 	cd "$D" &&
 	git clone . prune-tags-branch &&
 	cd prune-tags-branch &&
-	git fetch origin refs/heads/master:refs/remotes/origin/extrabranch &&
+	git update-ref refs/remotes/origin/extrabranch master &&
 
 	git fetch --prune --tags origin master &&
 	git rev-parse origin/extrabranch