Add a check to ensure that the all commits are Signed-off-by

This ensures that those using the Makefile to handle their git pushes to
Mahara confirm with the policy on submitting code at
http://wiki.mahara.org/index.php?title=Developer_Area/Contributing_CodeSigned-off-by: Andrew Robert Nicols <andrew.nicols@luns.net.uk>

Makefile

 all:
 	@echo "Run 'make imageoptim' to losslessly optimise all images"
 	@echo "Run 'make minaccept' to run the quick pre-commit tests"
+	@echo "Run 'make checksignoff' to check that your commits are all Signed-off-by"
 	@echo "Run 'make push' to push your changes to the repo"
 
 imageoptim:
@@ -12,5 +13,16 @@ imageoptim:
 minaccept:
 	@find htdocs/ -type f -name "*.php" | xargs -n 1 -P 2 php -l && echo All good!
 
+checksignoff:
+	@branch=`git status | head -1 | sed 's/.* On branch //'`; \
+	commits=`git log origin/$$branch.. | grep "^commit" | wc -l`; \
+	signed=`git log origin/$$branch.. | grep "Signed-off-by: " | wc -l`; \
+	if test "$$commits" -ne "$$signed"; then \
+		echo "$$(($$commits - $$signed)) commit(s) on $$branch not signed off ($$signed/$$commits)"; \
+		false; \
+	else \
+		echo "All commits signed!"; \
+	fi
+
 push: minaccept
 	git push