This gives users who build from the source tarball, but
do not want to install pandoc, access to the man pages.
Apperently the gocryptfs homebrew package ships without
the man pages at the moment to avoid pandoc.
Requested at
https://github.com/rfjakob/gocryptfs/issues/355
To be able to check out an older version and create
a tarball from it, let `git archive` operate on HEAD.
This used to be broken in a bad way: we use `git describe`
which operates on HEAD to name the tarball, but always archived
HEAD.
