Only create the archive file when it is done

cc @mr-vinn

the build is green on my machine (yes, lame)

mentioned in issue #3 (closed)

When the archive creation is interrupted, is it ok to leave the temporary archives in the filesystem? The old code used a Tempfile for this, which would take care of cleanup.

Status changed to closed

Closing in favor of another MR.

@mr-vinn yes, it is OK to leave the file because GitLab (the Rails app) cleans it up here: https://gitlab.com/gitlab-org/gitlab-ce/blob/7-3-stable/app/models/repository.rb#L231

Relying on cleanup from GitLab in the future after an aborted archive attempt is not ideal, but it is hard to avoid because the Unicorn worker is likely to get killed with SIGKILL (kill -9), leaving it no chance to run its own cleanup code.