Make GIT_REV logic work in release tarballs

Make GIT_REV logic work in release tarballs

Currently GIT_REV doesn't get set properly when building a release
tarball. To fix this we arrange for .gitcommit to contain the (short)
commit hash in tarballs generated with git-archive(1) using export-subst in
gitattributes. This way the correct commit hash is (reproducibly) included
in the release tarballs while not burdening the maintainers with updating
it in the git repo.

Please note this even works on Github and similar forges as they use
git-archive for generating tarballs so this works out quite nicely.