[test] Add target check-ref-dups that lists reference images having duplicate sha1sum hash

This is useful to review every once in a while to drop duplicate images that
can be dropped.  For example, if there are two identical images one named
some-test-svg-rgb24-ref.png and other some-test-svg-argb32-ref.png, those two
can be replaced with some-test-svg-ref.png.

test/.gitignore

@@ -3,6 +3,7 @@
 Makefile
 Makefile.in
 index.html
+ref.hash
 a8-mask
 bitmap-font
 caps-joins

test/Makefile.am

@@ -440,13 +440,8 @@ CLEANFILES =					\
 	*-diff.png				\
 	*.log					\
 	valgrind-log				\
-	multi-page.ps				\
-	multi-page.pdf				\
-	pdf-features.pdf			\
-	ps-features.ps				\
-	svg-surface.svg				\
-	svg-clip.svg				\
 	index.html				\
+	ref.hash				\
 	$(EXTRA_LTLIBRARIES)			\
 	$(EXTRA_PROGRAMS)
 
@@ -502,4 +497,9 @@ index.html: $(srcdir)/make-html.pl $(TESTS:=.log)
 	@echo Creating index.html
 	@perl $^ > $@
 
-.PHONY: check-valgrind test recheck retest html rehtml
+# Identify identical reference images
+check-ref-dups:
+	@sha1sum *-ref.png > ref.hash
+	@join ref.hash ref.hash | grep -v -E '( .*-ref.png).*\1' | cut -d' ' -f 1-2 | sort -u
+
+.PHONY: check-valgrind test recheck retest html rehtml check-ref-dups