repo-pull: Allow pulling only one directory

Changes the pull API to allow pulling only a single directory instead
of the whole deployment.  This option is utilized by the check-diff
option in rpm-ostree.

Add a new state directory to hold <checksum>.commitpartial files, so
we know that we've only downloaded partial state.

src/libostree/ostree-repo-private.h

@@ -40,6 +40,7 @@ struct OstreeRepo {
   GFile *local_heads_dir;
   GFile *remote_heads_dir;
   GFile *objects_dir;
+  GFile *state_dir;
   int objects_dir_fd;
   GFile *deltas_dir;
   GFile *uncompressed_objects_dir;