test: optionally test contrib in CI

Recently it was reported that a topic merged to 'next' broke build
and test for contrib/subtree part of the system.

Instead of having those who run 'next' or 'master' to hit the build
and test breakage and report to us, make sure we notice breakages in
contrib/ area before they hit my tree at all, during their own
presubmit testing.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -342,6 +342,9 @@ include shared.mak
 # If it isn't set, fallback to $LC_ALL, $LANG or use the first utf-8
 # locale returned by "locale -a".
 #
+# Define TEST_CONTRIB_TOO to make "make test" run tests in contrib/
+# directories.
+#
 # Define HAVE_CLOCK_GETTIME if your platform has clock_gettime.
 #
 # Define HAVE_CLOCK_MONOTONIC if your platform has CLOCK_MONOTONIC.
@@ -3369,6 +3372,9 @@ export TEST_NO_MALLOC_CHECK
 
 test: all
 	$(MAKE) -C t/ all
+ifdef TEST_CONTRIB_TOO
+	$(MAKE) -C contrib/ test
+endif
 
 perf: all
 	$(MAKE) -C t/perf/ all

ci/run-build-and-tests.sh

@@ -5,6 +5,8 @@
 
 . ${0%/*}/lib.sh
 
+export TEST_CONTRIB_TOO=yes
+
 case "$jobname" in
 fedora-breaking-changes-musl|linux-breaking-changes)
 	export WITH_BREAKING_CHANGES=YesPlease
@@ -36,6 +38,7 @@ linux-sha256)
 linux-reftable|linux-reftable-leaks|osx-reftable)
 	export GIT_TEST_DEFAULT_REF_FORMAT=reftable
 	;;
+
 esac
 
 case "$jobname" in

contrib/Makefile

@@ -0,0 +1,10 @@
+all::
+
+test::
+	$(MAKE) -C diff-highlight $@
+	$(MAKE) -C subtree $@
+
+clean::
+	$(MAKE) -C contacts $@
+	$(MAKE) -C diff-highlight $@
+	$(MAKE) -C subtree $@