pullrequests: optimize locating changeset to compare

We only need the max revision - that is much faster when a branch is specified.

rhodecode/model/pull_request.py

@@ -183,12 +183,12 b' class PullRequestModel(BaseModel):'
                     'rev': 'id',
                 }
 
-            org_rev_spec = "%s('%s')" % (_revset_predicates[org_ref[0]],
+            org_rev_spec = "max(%s('%s'))" % (_revset_predicates[org_ref[0]],
                                          safe_str(org_ref[1]))
             org_revs = scmutil.revrange(org_repo._repo, [org_rev_spec])
             org_rev = org_repo._repo[org_revs[-1] if org_revs else -1].hex()
 
-            other_rev_spec = "%s('%s')" % (_revset_predicates[other_ref[0]],
+            other_rev_spec = "max(%s('%s'))" % (_revset_predicates[other_ref[0]],
                                            safe_str(other_ref[1]))
             other_revs = scmutil.revrange(other_repo._repo, [other_rev_spec])
             other_rev = other_repo._repo[other_revs[-1] if other_revs else -1].hex()