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: I9a24a526e66f58ef2ebd9a2319bf96cf43e3e6f7
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Simo Fält <simo.falt@nokia.com>

.gitattributes

-.tag ident
+.tag                    export-subst
+.gitignore              export-ignore
+.gitattributes          export-ignore
+.commit-template        export-ignore

.tag

-$Id$
+$Format:%H$