.gitconfig: Use git-contacts instead of git-cc

Since Git 1.8.4, git has distributed Eric Sunshine's Perl port of
Felipe Contreras' cc-generating script.  Update to use the
newly-standardized script.  The new script is in the Git source under
contrib/contacts/.  Just drop it in your path.
.gitconfig: Enable rerere support

Re-merging and re-rebasing are not fun when you have to keep repeating
your previous conflict resolution.  See git-rerere(1) for details.