pullrequests: saving raw_id instead of branch in org_ref

This fixes an issue when the pull request is created while the top commit
included is not the tip. In that case, the faulty code would but
branch:default:default as org_ref, that causes the pull request view to show
all commits on that branch instead of the wanted commit(s) only. The fix will
result in a branch:default:1234567890abcdef1234567890abcdef (e.g)