ci/lib.sh: add support for Azure Pipelines

ci/lib.sh: add support for Azure Pipelines

This patch introduces a conditional arm that defines some environment
variables and a function that displays the URL given the job id (to
identify previous runs for known-good trees).

Because Azure Pipeline's macOS agents already have git-lfs and gettext
installed, we can leave `BREW_INSTALL_PACKAGES` empty (unlike in
Travis' case).

Note: this patch does not introduce an Azure Pipelines definition yet;
That is left for the next patch.

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