lib/repo: Create repo directories as 0775

For repo structure directories like `objects`, `refs`, etc... we should
be more permissive and let the system's `umask` narrow down the
permission bits as wanted.

This came up in a context where we want to be able to have read/write
access on an OSTree repo on NFS from two separate OpenShift apps by
using supplemental groups[1] so we don't require SCCs for running as the
same UID (supplemental groups are part of the default restricted SCC).

[1] https://docs.openshift.com/container-platform/3.11/install_config/persistent_storage/persistent_storage_nfs.html#nfs-supplemental-groups

src/libostree/ostree-repo-checkout.c

@@ -92,8 +92,8 @@ checkout_object_for_uncompressed_cache (OstreeRepo      *self,
 
   if (self->uncompressed_objects_dir_fd == -1)
     {
-      if (!glnx_shutil_mkdir_p_at (self->repo_dir_fd, "uncompressed-objects-cache", 0755,
-                                   cancellable, error))
+      if (!glnx_shutil_mkdir_p_at (self->repo_dir_fd, "uncompressed-objects-cache",
+                                   DEFAULT_DIRECTORY_MODE, cancellable, error))
         return FALSE;
       if (!glnx_opendirat (self->repo_dir_fd, "uncompressed-objects-cache", TRUE,
                            &self->uncompressed_objects_dir_fd,