lib/repo: Cleanup current boot's staging dir min-free-space-* checks are hit

min-free-space-* act as a gating condition whether to we want hold onto caches in
repo/tmp. If it is found that the free-disk space is going below this threshold,
we flag it as an error and cleanup current boot's staging directory.

Closes: #1602
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -152,6 +152,7 @@ struct OstreeRepo {
   gid_t target_owner_gid;
   guint min_free_space_percent; /* See the min-free-space-percent config option */
   guint64 min_free_space_mb; /* See the min-free-space-size config option */
+  gboolean cleanup_stagedir;
 
   guint test_error_flags; /* OstreeRepoTestErrorFlags */