Add git-archive export options

Add .gitattributes and .tag file to store the
sha1 of the commit being packaged

Change-Id: I1a880c5b0704c3a4f4e0643afc3296d48d3e3758
Reviewed-by: Jesus Fernandez <jesus.fernandez@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>