git-svn: track writes writes to the index in fetch

git-svn: track writes writes to the index in fetch

Introducing Git::IndexInfo.  This module will probably be useful
outside of git-svn, so I'm not putting it in the Git::SVN
namespace.

This will allow me to more easily avoid the use of get_log() in
the future and simply run do_update in incrementing ranges.
get_log() should be avoided because there are cases where
moved/deleted directories do not track correctly (until
--follow-parent is run on a new branch).

Signed-off-by: Eric Wong <normalperson@yhbt.net>