ci/lib.sh: encapsulate Travis-specific things

ci/lib.sh: encapsulate Travis-specific things

The upcoming patches will allow building git.git via Azure Pipelines
(i.e. Azure DevOps' Continuous Integration), where variable names and
URLs look a bit different than in Travis CI.

Also, the configurations of the available agents are different. For
example, Travis' and Azure Pipelines' macOS agents are set up
differently, so that on Travis, we have to install the git-lfs and
gettext Homebrew packages, and on Azure Pipelines we do not need to.
Likewise, Azure Pipelines' Ubuntu agents already have asciidoctor
installed.

Finally, on Azure Pipelines the natural way is not to base64-encode tar
files of the trash directories of failed tests, but to publish build
artifacts instead. Therefore, that code to log those base64-encoded tar
files is guarded to be Travis-specific.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>