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

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