ci: adds git fetch before doing schema checks

doc/Makefile

@@ -209,6 +209,8 @@ doc/index.rst: $(MANPAGES:=.md)
 
 # So GitHub renamed master to main.  This is painful.
 schema-added-check:
+	@git fetch origin master 2>&1; true
+	@git fetch origin main 2>&1 ; true
 	@if ! git describe origin/master >/dev/null 2>&1; then MASTER=main; else MASTER=master; fi; if git diff origin/$$MASTER doc/schemas | grep -q '^+.*{' && ! git diff origin/$$MASTER doc/schemas | grep -q '^+.*"added"'; then git diff origin/$$MASTER doc/schemas ; echo 'New schema fields must have "added": "vNEXTVERSION"' >&2; exit 1; fi
 
 # So GitHub renamed master to main.  This is painful.