GitLab

This commit was supposed to fix travis CI build failures, caused by the
fact that abuild didn't ignore the exit status of git-describe(1), but
didn't. A proper fix for this issue has been committed in
1b18665c.

This reverts commit cb3f79af.

https://docs.travis-ci.com/user/customizing-the-build/#Git-Clone-Depth

This reverts commit 70cd9559.

xenial is currently unstable, many builds fail for weird reasons or
don't even start.