Update the git-archive export options

More information: https://bugreports.qt-project.org/browse/QTBUG-33857



Change-Id: I667be8df624b1ab857e29443298c4ee29387dbcf
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>

.gitattributes

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

.tag

+$Format:%H$