Fix crash on $(PULL_REQUEST) expansion

by directly passing it to Ruby without passing a shell.
Formerly it was broken when $(PULL_REQUEST) included quotes.

defs/gmake.mk

@@ -192,9 +192,12 @@ checkout-github: fetch-github
 .PHONY: update-github
 update-github: checkout-github
 	$(eval PULL_REQUEST_API := https://api.github.com/repos/ruby/ruby/pulls/$(PR))
-	$(eval PULL_REQUEST := $(shell curl -s $(if $(GITHUB_TOKEN),-H "Authorization: bearer $(GITHUB_TOKEN)") $(PULL_REQUEST_API)))
-	$(eval FORK_REPO := $(shell $(BASERUBY) -rjson -e 'print JSON.parse(ARGV[0]).dig("head", "repo", "full_name")' '$(PULL_REQUEST)'))
-	$(eval PR_BRANCH := $(shell $(BASERUBY) -rjson -e 'print JSON.parse(ARGV[0]).dig("head", "ref")' '$(PULL_REQUEST)'))
+	$(eval PULL_REQUEST_FORK_BRANCH := $(shell \
+	  curl -s $(if $(GITHUB_TOKEN),-H "Authorization: bearer $(GITHUB_TOKEN)") $(PULL_REQUEST_API) | \
+	  $(BASERUBY) -rjson -e 'JSON.parse(STDIN.read)["head"].tap { |h| print "#{h["repo"]["full_name"]} #{h["ref"]}" }' \
+	))
+	$(eval FORK_REPO := $(shell echo $(PULL_REQUEST_FORK_BRANCH) | cut -d' ' -f1))
+	$(eval PR_BRANCH := $(shell echo $(PULL_REQUEST_FORK_BRANCH) | cut -d' ' -f2))
 	git merge master --no-edit
 	git remote add fork-$(PR) git@github.com:$(FORK_REPO).git
 	git push fork-$(PR) gh-$(PR):$(PR_BRANCH)