git-isvn(1): Integrated Subversion for Git.

git-isvn is a threaded C frontend for Git that pulls from Subversion
much faster than git-svn.perl, and most importantly, isn't written in
Perl.

WORKS:
  - clone (on some repositories -- in particular, svn branch points
    are broken at the moment).

TODO:
  - fetch/revmap
  - dcommit
  - continue to flesh out docs.