dependencies: dulwich, backported a patch to handle GIT references that are directories.

- present in dulwich 0.19.X

pkgs/patches/dulwich/handle-dir-refs.patch

@@ -0,0 +1,15 b''
+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
+

pkgs/python-packages-overrides.nix

@@ -46,6 +46,13 b' self: super: {'
     ];
   });
 
+  "dulwich" = super."dulwich".override (attrs: {
+    patches = [
+      ./patches/dulwich/handle-dir-refs.patch
+    ];
+  });
+
+
   # Avoid that base packages screw up the build process
   inherit (basePythonPackages)
     setuptools;