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)

1 files changed, 3 insertions, 3 deletions

diff --git a/libbe/storage/vcs/git.py b/libbe/storage/vcs/git.py
index 6d240c2..29abda7 100644
--- a/libbe/storage/vcs/git.py
+++ b/libbe/storage/vcs/git.py
@@ -143,12 +143,12 @@ class Git(base.VCS):
             if error.startswith("fatal: ambiguous argument 'HEAD': unknown "):
                 return None
             raise base.CommandError(args, status, stderr=error)
-        commits = output.splitlines()
+        revisions = output.splitlines()
         try:
             if index > 0:
-                return commits[index-1]
+                return revisions[index-1]
             elif index < 0:
-                return commits[index]
+                return revisions[index]
             else:
                 return None
         except IndexError: