fetch: treat --tags like refs/tags/*:refs/tags/* when pruning

If --tags is specified, add that refspec to the list given to
prune_refs so it knows to treat it as a filter on what refs to
should consider for prunning. This way

    git fetch --prune --tags origin

only prunes tags and doesn't delete the branch refs.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -735,11 +735,30 @@ static int do_fetch(struct transport *transport,
 		return 1;
 	}
 	if (prune) {
-		if (ref_count)
+		/* If --tags was specified, pretend the user gave us the canonical tags refspec */
+		if (tags == TAGS_SET) {
+			const char *tags_str = "refs/tags/*:refs/tags/*";
+			struct refspec *tags_refspec, *refspec;
+
+			/* Copy the refspec and add the tags to it */
+			refspec = xcalloc(ref_count + 1, sizeof(struct refspec));
+			tags_refspec = parse_fetch_refspec(1, &tags_str);
+			memcpy(refspec, refs, ref_count * sizeof(struct refspec));
+			memcpy(&refspec[ref_count], tags_refspec, sizeof(struct refspec));
+			ref_count++;
+
+			prune_refs(refspec, ref_count, ref_map);
+
+			ref_count--;
+			/* The rest of the strings belong to fetch_one */
+			free_refspec(1, tags_refspec);
+			free(refspec);
+		} else if (ref_count) {
 			prune_refs(refs, ref_count, ref_map);
-		else
+		} else {
 			prune_refs(transport->remote->fetch, transport->remote->fetch_refspec_nr, ref_map);
 		}
+	}
 	free_refs(ref_map);
 
 	/* if neither --no-tags nor --tags was specified, do automated tag

t/t5510-fetch.sh

@@ -105,7 +105,7 @@ test_expect_success 'fetch --prune with a namespace keeps other namespaces' '
 	git rev-parse origin/master
 '
 
-test_expect_failure 'fetch --prune --tags does not delete the remote-tracking branches' '
+test_expect_success 'fetch --prune --tags does not delete the remote-tracking branches' '
 	cd "$D" &&
 	git clone . prune-tags &&
 	cd prune-tags &&
@@ -116,7 +116,7 @@ test_expect_failure 'fetch --prune --tags does not delete the remote-tracking br
 	test_must_fail git rev-parse somebranch
 '
 
-test_expect_failure 'fetch --prune --tags with branch does not delete other remote-tracking branches' '
+test_expect_success 'fetch --prune --tags with branch does not delete other remote-tracking branches' '
 	cd "$D" &&
 	git clone . prune-tags-branch &&
 	cd prune-tags-branch &&