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>

ci/lib.sh

@@ -34,6 +34,11 @@ save_good_tree () {
 # successfully before (e.g. because the branch got rebased, changing only
 # the commit messages).
 skip_good_tree () {
+	if test "$TRAVIS_DEBUG_MODE" = true
+	then
+		return
+	fi
+
 	if ! good_tree_info="$(grep "^$(git rev-parse $CI_COMMIT^{tree}) " "$good_trees_file")"
 	then
 		# Haven't seen this tree yet, or no cached good trees file yet.