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 $n string, ask git-archive to store
the SHA-1 of the commit being packaged.

Change-Id: Id603b3776e114b993bd7b1f07dd3c2e95219cf1b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>