repo: Hold an fd "repo_dir_fd" open for the toplevel too

We could just make everything relative to this, but the objects/ and
tmp/ are accessed very often, so I think it's worth holding individual
fds.

This fd can cover everything else: refs, deltas, etc.

src/libostree/ostree-repo-private.h

@@ -37,6 +37,7 @@ struct OstreeRepo {
   char *boot_id;
 
   GFile *repodir;
+  int    repo_dir_fd;
   GFile *tmp_dir;
   int    tmp_dir_fd;
   GFile *local_heads_dir;