t5510: check that "git fetch --prune --tags" does not prune branches

"git fetch --prune --tags" is currently interpreted as follows:

* "--tags" is equivalent to specifying a refspec
  "refs/tags/*:refs/tags/*", and supersedes any default refspecs
  configured via remote.$REMOTE.fetch.

* "--prune" only operates on the refspecs being fetched.

Therefore, "git fetch --prune --tags" prunes tags in refs/tags/* but
does not fetch or prune other references.  The fact that this command
does not prune references outside of refs/tags/* was previously
untested.  So add a test that verifies the status quo.

However, the status quo is surprising, so it will be changed later in
this patch series.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5510-fetch.sh

@@ -118,9 +118,13 @@ test_expect_success 'fetch --prune --tags does not delete the remote-tracking br
 	git clone . prune-tags &&
 	cd prune-tags &&
 	git tag sometag master &&
+	# Create what looks like a remote-tracking branch from an earlier
+	# fetch that has since been deleted from the remote:
+	git update-ref refs/remotes/origin/fake-remote master &&
 
 	git fetch --prune --tags origin &&
 	git rev-parse origin/master &&
+	git rev-parse origin/fake-remote &&
 	test_must_fail git rev-parse sometag
 '