make erlang-mk: Copy build.config after switching to $(ERLANG_MK_COMMIT)

If the local build.config has changes compared to upstream's
master branch, and $(ERLANG_MK_COMMIT) also brings changes to this
file, copying the local file to the checkout before switching to
$(ERLANG_MK_COMMIT) caused git-checkout(1) to abort with:

    error: Your local changes to the following files would be overwritten by checkout:
    	build.config
    Please, commit your changes or stash them before you can switch branches.
    Aborting

While here, fix `core-upgrade-custom-repo` test case: without switching
back the alternate erlang.mk repository to its master branch, the clone
always got the test-copyright branch by default, making the
ERLANG_MK_COMMIT check ineffective.

core/core.mk

@@ -192,8 +192,11 @@ ERLANG_MK_BUILD_DIR ?= .erlang.mk.build
 
 erlang-mk:
 	git clone $(ERLANG_MK_REPO) $(ERLANG_MK_BUILD_DIR)
+ifdef ERLANG_MK_COMMIT
+	cd $(ERLANG_MK_BUILD_DIR) && git checkout $(ERLANG_MK_COMMIT)
+endif
 	if [ -f $(ERLANG_MK_BUILD_CONFIG) ]; then cp $(ERLANG_MK_BUILD_CONFIG) $(ERLANG_MK_BUILD_DIR)/build.config; fi
-	cd $(ERLANG_MK_BUILD_DIR) && $(if $(ERLANG_MK_COMMIT),git checkout $(ERLANG_MK_COMMIT) &&) $(MAKE)
+	$(MAKE) -C $(ERLANG_MK_BUILD_DIR)
 	cp $(ERLANG_MK_BUILD_DIR)/erlang.mk ./erlang.mk
 	rm -rf $(ERLANG_MK_BUILD_DIR)
 

test/core_upgrade.mk

@@ -1,6 +1,6 @@
 # Core: Erlang.mk upgrade.
 
-CORE_UPGRADE_CASES = custom-build-dir custom-config custom-repo no-config renamed-config
+CORE_UPGRADE_CASES = conflicting-configs custom-build-dir custom-config custom-repo no-config renamed-config
 CORE_UPGRADE_TARGETS = $(addprefix core-upgrade-,$(CORE_UPGRADE_CASES))
 CORE_UPGRADE_CLEAN_TARGETS = $(addprefix clean-,$(CORE_UPGRADE_TARGETS))
 
@@ -13,6 +13,35 @@ $(CORE_UPGRADE_CLEAN_TARGETS):
 
 core-upgrade: $(CORE_UPGRADE_TARGETS)
 
+core-upgrade-conflicting-configs: build clean-core-upgrade-conflicting-configs
+
+	$i "Bootstrap a new OTP library named $(APP)"
+	$t mkdir $(APP)/
+	$t cp ../erlang.mk $(APP)/
+	$t $(MAKE) -C $(APP) -f erlang.mk bootstrap-lib $v
+
+	$i "Fork erlang.mk locally and modify it"
+	$t git clone -q https://github.com/ninenines/erlang.mk $(APP)/alt-erlangmk-repo
+	$t echo core/core > $(APP)/alt-erlangmk-repo/build.config
+	$t (cd $(APP)/alt-erlangmk-repo && \
+		git checkout -q -b test-modified-build.config && \
+		git config user.email "testsuite@erlang.mk" && \
+		git config user.name "test suite" && \
+		git commit -q -a -m 'Modify build.config' && \
+		git checkout master)
+
+	$i "Point application to an alternate erlang.mk repository"
+	$t perl -ni.bak -e 'print;if ($$.==1) {print "ERLANG_MK_REPO = file://$(abspath $(APP)/alt-erlangmk-repo)\nERLANG_MK_COMMIT = test-modified-build.config\n"}' $(APP)/Makefile
+
+	$i "Create a custom build.config file without plugins"
+	$t echo "core/*" > $(APP)/build.config
+
+	$i "Upgrade Erlang.mk"
+	$t $(MAKE) -C $(APP) erlang-mk $v
+
+	$i "Check that the bootstrap plugin is gone"
+	$t ! $(MAKE) -C $(APP) list-templates $v
+
 core-upgrade-custom-build-dir: build clean-core-upgrade-custom-build-dir
 
 	$i "Bootstrap a new OTP library named $(APP)"
@@ -69,7 +98,8 @@ core-upgrade-custom-repo: build clean-core-upgrade-custom-repo
 		git checkout -q -b test-copyright && \
 		git config user.email "testsuite@erlang.mk" && \
 		git config user.name "test suite" && \
-		git commit -q -a -m 'Add Testsuite copyright')
+		git commit -q -a -m 'Add Testsuite copyright' && \
+		git checkout master)
 
 	$i "Point application to an alternate erlang.mk repository"
 	$t perl -ni.bak -e 'print;if ($$.==1) {print "ERLANG_MK_REPO = file://$(abspath $(APP)/alt-erlangmk-repo)\nERLANG_MK_COMMIT = test-copyright\n"}' $(APP)/Makefile