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-pull.c

@@ -2925,7 +2925,7 @@ _ostree_repo_cache_summary (OstreeRepo        *self,
   if (self->cache_dir_fd == -1)
     return TRUE;
 
-  if (!glnx_shutil_mkdir_p_at (self->cache_dir_fd, _OSTREE_SUMMARY_CACHE_DIR, 0775, cancellable, error))
+  if (!glnx_shutil_mkdir_p_at (self->cache_dir_fd, _OSTREE_SUMMARY_CACHE_DIR, DEFAULT_DIRECTORY_MODE, cancellable, error))
     return FALSE;
 
   const char *summary_cache_file = glnx_strjoina (_OSTREE_SUMMARY_CACHE_DIR, "/", remote);