Must tag desired commit when fetching

Fixes #238

Author: devrandom

bin/gbuild

@@ -318,8 +318,8 @@ build_desc["remotes"].each do |remote|
     system!("git init inputs/#{dir}")
   end
   if !@options[:skip_fetch]
-    system!("cd inputs/#{dir} && git fetch --update-head-ok #{sanitize_path(remote["url"], remote["url"])} #{commit}")
-    system!("cd inputs/#{dir} && git checkout -q #{commit}")
+    system!("cd inputs/#{dir} && git fetch --update-head-ok #{sanitize_path(remote["url"], remote["url"])} #{commit}:refs/tags/tobuild")
+    system!("cd inputs/#{dir} && git checkout -q tobuild")
     system!("cd inputs/#{dir} && git submodule update --init --recursive --force")
   end
   commit = `cd inputs/#{dir} && git log --format=%H -1 #{commit}`.strip