Update the git-archive export options

The .gitattributes, .gitignore, .commit-template 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: I46c0496d0a58c2b77d7c662a705c0ac68005b02c
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>