Split SOURCE_REF into its own makefile function

Author: José Valim

Makefile

@@ -94,11 +94,13 @@ clean:
 
 #==> Release tasks
 
+SOURCE_REF = $(shell head="$$(git rev-parse HEAD)" tag="$$(git tag --points-at $$head | tail -1)" ; echo "$${tag:-$$head}\c")
+
 docs: compile
 	mkdir -p ebin
 	rm -rf docs
 	cp -R -f lib/*/ebin/*.beam ./ebin
-	bin/elixir ../exdoc/bin/exdoc "Elixir" "$(VERSION)" -m Kernel -u "https://github.com/elixir-lang/elixir" --source-ref "$(shell head="$$(git rev-parse HEAD)" tag="$$(git tag --points-at $$head | tail -1)" ; echo "$${tag:-$$head}\c")"
+	bin/elixir ../exdoc/bin/exdoc "Elixir" "$(VERSION)" -m Kernel -u "https://github.com/elixir-lang/elixir" --source-ref "$(call SOURCE_REF)"
 	rm -rf ebin
 
 release_zip: compile