Merge branch 'dw/diff-highlight-makefile-fix' into maint

Build clean-up.

* dw/diff-highlight-makefile-fix:
  diff-highlight: add clean target to Makefile

Author: gitster

contrib/diff-highlight/Makefile

@@ -17,4 +17,7 @@ shebang.perl: FORCE
 test: all
 	$(MAKE) -C t
 
+clean:
+	$(RM) diff-highlight
+
 .PHONY: FORCE