Add git revision to version number only if source is a git repo

Check if the source folder is a git working tree by looking for a .git
file or folder instead of relying solely on git rev-parse. The latter
will also succeed if the source tarball is extracted inside an unrelated
git working tree (as we did with gpg4win 4.x).

This change also removes the kleopatra_release variable which is long
obsolete.