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: I0c93d28c4a96d4479f141c18f3234e5252be6d7c
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

.gitattributes

+.tag            export-subst
+.gitignore      export-ignore
+.gitattributes  export-ignore

.tag

+$Format:%H$