sequencer: beautify subject of reverts of reverts

sequencer: beautify subject of reverts of reverts

Instead of generating a silly-looking `Revert "Revert "foo""`, make it
a more humane `Reapply "foo"`.

This is done for two reasons:
- To cover the actually common case of just a double revert.
- To encourage people to rewrite summaries of recursive reverts by
  setting an example (a subsequent commit will also do this explicitly
  in the documentation).

To achieve these goals, the mechanism does not need to be particularly
sophisticated. Therefore, more complicated alternatives which would
"compress more efficiently" have not been implemented.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>