Do test build after merge on pull request - not just from source branch

As a continuous integration admin I need to have option to do test build for pull request not from source branch but from source code after merge before I can allow pull request on stash.

Currently pull request trigger test builds on source branch, but sometimes source branch build could be successful, but when code is merged to destination branch all builds start failing because some other merge to destination branch was made before and code is inconsistent state.

It is really useful when I do pull request, Bamboo will do local merge of source and destination branch and then trigger build on this merged state. If this build is not successful then it will not allow pull request merge in stash.