core: Add dynamic uncompressed object cache for archive-z

This gives us something closer to the advantages of archive and
archive-z when using the latter.  Concretely we get deduplication
among multiple checkouts, along with the "devino" hash table trick
during commits to avoid checksumming content again.

This is enabled by default.

src/libotutil/ot-gio-utils.c

@@ -57,6 +57,7 @@ ot_gfile_ensure_directory (GFile     *dir,
   gboolean ret = FALSE;
   GError *temp_error = NULL;
 
+ again:
   if (with_parents)
     ret = g_file_make_directory_with_parents (dir, NULL, &temp_error);
   else
@@ -71,6 +72,11 @@ ot_gfile_ensure_directory (GFile     *dir,
       else
         g_clear_error (&temp_error);
     }
+  /* Work around glib bug where if multiple threads/processes race in
+   * _with_parents, it can error out early
+   */
+  if (with_parents && !g_file_query_exists (dir, NULL))
+    goto again;
 
   ret = TRUE;
  out: