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>

ci/run-build-and-tests.sh

@@ -52,4 +52,8 @@ then
 fi
 check_unignored_build_artifacts
 
+case " $MAKE_TARGETS " in
+*" all "*) make -C contrib/subtree test;;
+esac
+
 save_good_tree

ci/run-test-slice.sh

@@ -15,4 +15,7 @@ group "Run tests" make --quiet -C t T="$(cd t &&
 	tr '\n' ' ')" ||
 handle_failed_tests
 
+# Run the git subtree tests only if main tests succeeded
+test 0 != "$1" || make -C contrib/subtree test
+
 check_unignored_build_artifacts