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

@@ -1,53 +1,60 b''
 # Overrides for the generated python-packages.nix
 #
 # This function is intended to be used as an extension to the generated file
 # python-packages.nix. The main objective is to add needed dependencies of C
 # libraries and tweak the build instructions where needed.
 
 { pkgs
 , basePythonPackages
 }:
 
 let
   sed = "sed -i";
 
 in
 
 self: super: {
 
   "gevent" = super."gevent".override (attrs: {
     propagatedBuildInputs = attrs.propagatedBuildInputs ++ [
       # NOTE: (marcink) odd requirements from gevent aren't set properly,
       # thus we need to inject psutil manually
       self."psutil"
     ];
   });
 
   "hgsubversion" = super."hgsubversion".override (attrs: {
     propagatedBuildInputs = attrs.propagatedBuildInputs ++ [
       pkgs.sqlite
       #basePythonPackages.sqlite3
       self.mercurial
     ];
   });
 
   "subvertpy" = super."subvertpy".override (attrs: {
     SVN_PREFIX = "${pkgs.subversion.dev}";
     propagatedBuildInputs = [
       pkgs.apr.dev
       pkgs.aprutil
       pkgs.subversion
     ];
   });
 
   "mercurial" = super."mercurial".override (attrs: {
     propagatedBuildInputs = [
       # self.python.modules.curses
     ];
   });
 
+  "dulwich" = super."dulwich".override (attrs: {
+    patches = [
+      ./patches/dulwich/handle-dir-refs.patch
+    ];
+  });
+
+
   # Avoid that base packages screw up the build process
   inherit (basePythonPackages)
     setuptools;
 
 }