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: I2b23ec88ebef02b059f56110eaca5e7c2895c791
Reviewed-by: Matti Paaso <matti.paaso@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>