travis-ci: use 'set -x' in 'ci/*' scripts for extra tracing output

While the build logic was embedded in our '.travis.yml', Travis CI used to produce a nice trace log including all commands executed in those embedded scriptlets. Since 657343a60 (travis-ci: move Travis CI code into dedicated scripts, 2017-09-10), however, we only see the name of the dedicated scripts, but not what those scripts are actually doing, resulting in a less useful trace log. A patch later in this series will move setting environment variables from '.travis.yml' to the 'ci/*' scripts, so not even those will be included in the trace log. Use 'set -x' in 'ci/lib-travisci.sh', which is sourced in most other 'ci/*' scripts, so we get trace log about the commands executed in all of those scripts. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>