Update the git-archive export options

The .gitattributes and .gitignore files do not need to be present in
packaged sources, as they are for people using the Git repository.
People who download tarballs usually don't reimport into Git -- they
should just download the actual repositories.

Meanwhile, the .tag file is quite useful. Instead of making Git extract
to the useless SHA-1 of the "$Id$\n" string, ask git-archive to store
the SHA-1 of the commit being packaged.

Change-Id: I5850800f9706dda342596a9db383e9d2c60e3c3e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>