The reason we don't run |git checkout --detach| in all cases is that it
fails when we cloned a repository without a master branch. To avoid
running it in this case, we were checking if HEAD is pointing to a
branch which has a corresponding file in the .git directory.

However it seems there are other cases where HEAD is pointing to a
branch which does not have a corresponding file in the .git directory.
Instead we now check that |git rev-parse --verify HEAD| does not return
an error, in which case we assume that |git checkout --detach| will not
produce an error too.

Set DEBIAN_FRONTEND=noninteractive when using apt-get in install_package.
Also use the -q (quiet) argument.

We replace a call to:
  git submodule update --init
with calls to:
  git submodule init
  git submodule sync
  git submodule update

The call to `git submodule sync` is solving the issue that could occur
when a submodule URL is changing.

To create an archive containing all submodules, we were creating a
temporary archive of each submodule before appending them to the main
archive. We were using the submodule path in the temporary archive
filename, which was failing if the submodule is in a subdirectory.
To avoid that, we are removing the path from the temporary archive's
filename and directly appending it to the main archive.

By default gpg will from time to time update its trust database. When
this happens it will also modify the keyring files to add some trust
information. To avoid this we add the --no-auto-check-trustdb option.

As we don't use the Web of Trust when we use a keyring file, we also
disable it using `--trust-model always`.

Print stderr from the git/hg command if the checkout failed.

This script will create a tarball containing the website and print
commands that can be used to upload it.