Fix diverging commit count calculation

Rugged seemed to stop accepting branch names as valid refs, throwing `Rugged::ReferenceError` exceptions. Now the branch target rather than branch name is used, and the default branch's hash is also calculated, and these work properly.

This used to work and might be something worth re-investigating in the future.

app/models/repository.rb

@@ -157,9 +157,12 @@ class Repository
   end
   
   def diverging_commit_counts(branch)
+    root_ref_hash = raw_repository.rev_parse_target(root_ref).oid
     cache.fetch(:"diverging_commit_counts_#{branch.name}") do
-      number_commits_behind = commits_between(branch.name, root_ref).size
-      number_commits_ahead = commits_between(root_ref, branch.name).size
+      # Rugged seems to throw a `ReferenceError` when given branch_names rather
+      # than SHA-1 hashes
+      number_commits_behind = commits_between(branch.target, root_ref_hash).size
+      number_commits_ahead = commits_between(root_ref_hash, branch.target).size
       
       { behind: number_commits_behind, ahead: number_commits_ahead }
     end