fix broken combined queues

bors.rust-lang.org mentions

> Homu provides a few simple ways to customize the queue's contents to fit your needs:
>
> * queue/rust+cargo will combine the queues of the rust and cargo repos (for example).

but it appears 963762a broke this by early aborting to a 404 without checking this case

homu/server.py

@@ -117,7 +117,10 @@ def result(repo_label, pull):
 
 @get('/queue/<repo_label:path>')
 def queue(repo_label):
-    if repo_label not in g.cfg['repo'] and repo_label != 'all':
+
+    labels = repo_label.split('+')
+    if repo_label != 'all' \
+            and any(label not in g.cfg['repo'] for label in labels):
         abort(404)
 
     logger = g.logger.getChild('queue')
@@ -126,19 +129,20 @@ def queue(repo_label):
 
     single_repo_closed = None
     treeclosed_src = None
+    repo_url = None
+
     if repo_label == 'all':
         labels = g.repos.keys()
         multiple = True
-        repo_url = None
     else:
-        labels = repo_label.split('+')
         multiple = len(labels) > 1
-        if repo_label in g.repos and g.repos[repo_label].treeclosed >= 0:
-            single_repo_closed = g.repos[repo_label].treeclosed
-            treeclosed_src = g.repos[repo_label].treeclosed_src
-        repo_url = 'https://github.com/{}/{}'.format(
-            g.cfg['repo'][repo_label]['owner'],
-            g.cfg['repo'][repo_label]['name'])
+        if not multiple:
+            if repo_label in g.repos and g.repos[repo_label].treeclosed >= 0:
+                single_repo_closed = g.repos[repo_label].treeclosed
+                treeclosed_src = g.repos[repo_label].treeclosed_src
+            repo_url = 'https://github.com/{}/{}'.format(
+                g.cfg['repo'][repo_label]['owner'],
+                g.cfg['repo'][repo_label]['name'])
 
     states = []
     for label in labels: