travis-ci: do not skip successfully tested trees in debug mode

Travis CI offers shell access to its virtual machine environment running the build jobs, called "debug mode" [1]. After restarting a build job in debug mode and logging in, the first thing I usually do is to install dependencies, i.e. run './ci/install-dependencies.sh'. This works just fine when I restarted a failed build job in debug mode. However, after restarting a successful build job in debug mode our CI scripts get all clever, and exit without doing anything useful, claiming that "This commit's tree has already been built and tested successfully" [2]. Our CI scripts are right, and we do want to skip building and testing already known good trees in "regular" CI builds. In debug mode, however, this is a nuisiance, because one has to delete the cache (or at least the 'good-trees' file in the cache) to proceed. Let's update our CI scripts, in particular the common 'ci/lib.sh', to not skip previously successfully built and tested trees in debug mode, so all those scripts will do what there were supposed to do even when a successful build job was restarted in debug mode. [1] https://docs.travis-ci.com/user/running-build-in-debug-mode/ [2] 9cc2c76f5e (travis-ci: record and skip successfully built trees, 2017-12-31) Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>