Passing "-a" will make it easier to automatically create archives from
tagged repositories. It doesn't ask any question and doesn't return an
error when the current branch is not tagged nor if the release already
exists.
First we must not report an error when "git diff HEAD" fails. Second, we
don't want to "cd" to the home dir when "git rev-parse --show-toplevel"
returns an empty string. Third, we definitely want to check that a master
branch really exists in the current directory to avoid mistakes.
These ones have been used for several months already and are quite
convenient to emit new releases and backport fixes. I'm fed up with
having different versions on different machines, let's commit them
now.