Use commit finder instead of rev parse

This has the side effect of making this method rugged call free, which
is the reason I actually changed this.

app/models/repository.rb

@@ -256,7 +256,7 @@ class Repository
   end
   def diverging_commit_counts(branch)
-    root_ref_hash = raw_repository.rev_parse_target(root_ref).oid
+    root_ref_hash = raw_repository.commit(root_ref).id
     cache.fetch(:"diverging_commit_counts_#{branch.name}") do
       # Rugged seems to throw a `ReferenceError` when given branch_names rather
       # than SHA-1 hashes