Move Darcs over to Storage format.

Move Darcs over to Storage format.

We have to work around the same issue as mercurial (hg) issue 618.  I
can't find a Darcs bug report for this, but it's been fixed somewhere
between 1.0.9 and 2.3.1.

Example scripts demonstrating the bug:
  $ darcs=/usr/bin/darcs
  $ mkdir x; cd x; $darcs init; echo a > b; $darcs add b; \
    $darcs record --all --author 'x <a@b.com>' --logfile b; \
    echo z>b; $darcs record --all --author 'x <a@b.com>' --logfile b; \
    echo g>b; $darcs record --all --author 'x <a@b.com>' --logfile b; \
    cd ..; rm -rf x > /dev/null; $darcs --version
  Finished recording patch 'a'
  No changes!
  No changes!
  1.0.9 (release)
And showing it's been fixed:
  $ darcs=~/.cabal/bin/darcs
  $ mkdir x; cd x; $darcs init; echo a > b; $darcs add b; \
    $darcs record --all --author 'x <a@b.com>' --logfile b; \
    echo z>b; $darcs record --all --author 'x <a@b.com>' --logfile b; \
    echo g>b; $darcs record --all --author 'x <a@b.com>' --logfile b; \
    cd ..; rm -rf x > /dev/null; $darcs --version
  Finished recording patch 'a'
  Finished recording patch 'z'
  Finished recording patch 'g'
  2.3.1 (release)