contrib: wire up osxkeychain in contrib/Makefile on macOS

When running "make test" with TEST_CONTRIB_TOO=yes (which is default in
macOS CI workflows), $(MAKE) -C contrib/ test is invoked. However,
contrib/Makefile only invoked tests for diff-highlight and subtree,
meaning git-credential-osxkeychain was never built or verified during
standard CI test runs.

Add a "test" target to contrib/credential/osxkeychain/Makefile that
depends on building git-credential-osxkeychain. Additionally, wire up
credential/osxkeychain in contrib/Makefile under "all", "test", and
"clean" whenever running on macOS (Darwin).

This ensures that running "make test" or "make all" in contrib on macOS
automatically builds and links git-credential-osxkeychain, preventing
future build or symbol linking regressions from slipping through CI.

Signed-off-by: Shardul Natu <snatu@google.com>

Author: Shardul Natu

contrib/Makefile

@@ -1,10 +1,20 @@
+-include ../config.mak.autogen
+-include ../config.mak
+
+ifeq ($(uname_S),Darwin)
+OS_CONTRIB += credential/osxkeychain
+endif
+
 all::
+	$(foreach dir,$(OS_CONTRIB),$(MAKE) -C $(dir) $@;)
 
 test::
 	$(MAKE) -C diff-highlight $@
 	$(MAKE) -C subtree $@
+	$(foreach dir,$(OS_CONTRIB),$(MAKE) -C $(dir) $@;)
 
 clean::
 	$(MAKE) -C contacts $@
 	$(MAKE) -C diff-highlight $@
 	$(MAKE) -C subtree $@
+	$(foreach dir,$(OS_CONTRIB),$(MAKE) -C $(dir) $@;)

contrib/credential/osxkeychain/Makefile

@@ -10,4 +10,6 @@ install:
 clean:
 	$(MAKE) -C ../../.. clean-git-credential-osxkeychain
 
-.PHONY: all git-credential-osxkeychain install clean
+test: git-credential-osxkeychain
+
+.PHONY: all git-credential-osxkeychain install clean test