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

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>