Must change how branch testing is done

The other changes in the file, involving when the variables are
evaluated, meant that the use of Make's ifneq conditional no longer
worked as expected.

Makefile

@@ -150,9 +150,7 @@ run-shiv:
 release: | test-branch release-on-github print-instructions
 
 test-branch: vars
-ifneq ($(branch),main)
-	$(error Current git branch != main. Merge changes into main first)
-endif
+	$(if $(filter-out $(branch),main),$(error Current git branch != main.))
 
 update-init: vars
 	@sed -i .bak -e "s|^\(__version__ *=\).*|\1 '$(version)'|"  $(init_file)