Fix galaxy.util.in_directory() - broken because it takes realpath of …

…just one argument.

Easy to see on Mac OS X since /tmp isn't really /tmp for instance:

```
% touch /tmp/moo
% python
>>> import galaxy.util
>>> galaxy.util.in_directory("/tmp/moo", "/tmp/moo")
False
```

... and this is why the Travis tests are failing under Mac OS X. Broken with 93a8bfc.

lib/galaxy/util/path/__init__.py

@@ -119,7 +119,7 @@ def __walk(path):
 
 def __contains(prefix, path, whitelist=None):
     real = realpath(join(prefix, path))
-    yield not relpath(real, prefix).startswith(pardir)
+    yield not relpath(real, realpath(prefix)).startswith(pardir)
     for wldir in whitelist or []:
         yield not relpath(real, wldir).startswith(pardir)