Reference HEAD in Makefile (more portable than head)

This fixes "fatal: ambiguous argument 'head'", which occurs on some
systems, inclding GNU/Linux systems, with "git rev-parse head".

Author: EliahKagan

Makefile

@@ -9,9 +9,9 @@ clean:
 release: clean
 	# Check if latest tag is the current head we're releasing
 	echo "Latest tag = $$(git tag | sort -nr | head -n1)"
-	echo "HEAD SHA       = $$(git rev-parse head)"
+	echo "HEAD SHA       = $$(git rev-parse HEAD)"
 	echo "Latest tag SHA = $$(git tag | sort -nr | head -n1 | xargs git rev-parse)"
-	@test "$$(git rev-parse head)" = "$$(git tag | sort -nr | head -n1 | xargs git rev-parse)"
+	@test "$$(git rev-parse HEAD)" = "$$(git tag | sort -nr | head -n1 | xargs git rev-parse)"
 	make force_release
 
 force_release: clean