Replace getrev.sh with a one-liner (thanks @bellthoven)

Author: alco

Makefile

@@ -98,7 +98,7 @@ docs: compile
 	mkdir -p ebin
 	rm -rf docs
 	cp -R -f lib/*/ebin/*.beam ./ebin
-	bin/elixir ../exdoc/bin/exdoc "Elixir" "$(VERSION)" -m Kernel -u "https://github.com/elixir-lang/elixir" --source-ref `sh getrev.sh`
+	bin/elixir ../exdoc/bin/exdoc "Elixir" "$(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")"
 	rm -rf ebin
 
 release_zip: compile