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-refs.c

@@ -1181,7 +1181,7 @@ _ostree_repo_write_ref (OstreeRepo                 *self,
           char *parent = strdupa (ref->ref_name);
           parent[lastslash - ref->ref_name] = '\0';
 
-          if (!glnx_shutil_mkdir_p_at (dfd, parent, 0755, cancellable, error))
+          if (!glnx_shutil_mkdir_p_at (dfd, parent, DEFAULT_DIRECTORY_MODE, cancellable, error))
             return FALSE;
         }