git-svn: prevent dcommitting if the index is dirty.
dcommit uses rebase to sync the history with what has just been pushed to
SVN. Trying to dcommit with a dirty index is troublesome for rebase, so now
the user will get an error message if he attempts to dcommit with a dirty
index.
Signed-off-by: Benoit Sigoure <tsuna@lrde.epita.fr> Acked-by: Eric Wong <normalperson@yhbt.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>