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, if you're careful)

TODO:
  - fetch (resumed clone) / persisted revmap
  - dcommit
  - continue to flesh out docs.
git-isvn(1): Start porting from git to libgit2.

It builds! Ship it.

Minor details:

- Build git-isvn(1)/git(1) with -fvisibility=hidden so libgit2 doesn't
  try and call main binary symbols of the same name that don't do what
  it expects
- Checkout (post-clone) is mostly ported over to libgit2. Some details
  missing.