Use Repository#rev_parse_oid in Repository#create_branch

This is the purpose-built API to lookup an arbitrary treeish, and since
git_lookup_reference() returns a new error code, it messed up the original two-stage
lookup.

lib/rugged/repository.rb

@@ -140,11 +140,7 @@ def create_branch(name, sha_or_ref = "HEAD")
       when Rugged::Object
         target = sha_or_ref.oid
       else
-        if (ref = Rugged::Reference.lookup(self, sha_or_ref))
-          target = ref.resolve.target
-        else
-          target = Rugged::Commit.lookup(self, sha_or_ref)
-        end
+        target = rev_parse_oid(sha_or_ref)
       end
 
       Branch.create(self, name, target)