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

@@ -51,4 +51,8 @@ esac
 make $MAKE_TARGETS
 check_unignored_build_artifacts
 
+case " $MAKE_TARGETS " in
+*" all "*) make -C contrib/subtree test;;
+esac
+
 save_good_tree

ci/run-test-slice.sh

@@ -14,4 +14,7 @@ make --quiet -C t T="$(cd t &&
 	./helper/test-tool path-utils slice-tests "$1" "$2" t[0-9]*.sh |
 	tr '\n' ' ')"
 
+# Run the git subtree tests only if main tests succeeded
+test 0 != "$1" || make -C contrib/subtree test
+
 check_unignored_build_artifacts