This patch allows handling directories inside the refs/heads. This was added in dulwich
0.19.X series

diff -rup dulwich-0.13.0-orig/dulwich/refs.py dulwich-0.13.0/dulwich/refs.py
--- dulwich-0.13.0-orig/dulwich/refs.py	2018-10-09 09:42:38.182597268 +0200
+++ dulwich-0.13.0/dulwich/refs.py	2018-10-09 09:43:39.057145566 +0200
@@ -509,7 +509,7 @@ 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