git-svn-ahead can't handle empty commit messages

[scy]

If there are empty commit messages in SVN, git-svn-ahead's git log --pretty=format:'%b%n%H' will not output the git-svn-id: lines for these commits, because they are considered to be the subject (%s), not part of the body (%b).

The effect is that the ahead count will be computed wrongly if the empty commit message(s) is/are the most recent commit(s).

I should probably rewrite the parsing completely, it's too hackish right now.