Handle probes that hit directories.

dulwich/refs.py

@@ -564,7 +564,7 @@ class DiskRefsContainer(RefsContainer):
                     # Read only the first 40 bytes
                     return header + f.read(40 - len(SYMREF))
         except IOError as e:
-            if e.errno == errno.ENOENT:
+            if e.errno in (errno.ENOENT, errno.EISDIR):
                 return None
             raise