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: I44640a05f6e449f5d609d63f82a75fb3bce990e6
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>