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-static-delta-compilation.c

@@ -1427,7 +1427,7 @@ ostree_repo_static_delta_generate (OstreeRepo                   *self,
       g_autofree char *dnbuf = g_strdup (descriptor_relpath);
       const char *dn = dirname (dnbuf);
 
-      if (!glnx_shutil_mkdir_p_at (self->repo_dir_fd, dn, 0755, cancellable, error))
+      if (!glnx_shutil_mkdir_p_at (self->repo_dir_fd, dn, DEFAULT_DIRECTORY_MODE, cancellable, error))
         goto out;
       if (!glnx_opendirat (self->repo_dir_fd, dn, TRUE, &descriptor_dfd, error))
         goto out;