ci: run contrib/subtree tests in CI builds

Because `git subtree` (unlike most other `contrib` modules) is included as
part of the standard release of Git for Windows, its stability should be
verified as consistently as it is for the rest of git. By including the
`git subtree` tests in the CI workflow, these tests are as much of a gate to
merging and indicator of stability as the standard test suite.

Signed-off-by: Victoria Dye <vdye@github.com>

Author: vdye

Diff for: ci/run-build-and-tests.sh

@@ -73,5 +73,9 @@ case "$jobname" in
 	;;
 esac
 
+case " $MAKE_TARGETS " in
+*" all "*) make -C contrib/subtree test;;
+esac
+
 check_unignored_build_artifacts
 save_good_tree

Diff for: ci/run-test-slice.sh

@@ -20,4 +20,7 @@ if [ "$1" == "0" ] ; then
 	group "Run unit tests" make --quiet -C t unit-tests-test-tool
 fi
 
+# Run the git subtree tests only if main tests succeeded
+test 0 != "$1" || make -C contrib/subtree test
+
 check_unignored_build_artifacts