Add option for not including individual commit messages in pull request merge commit messages

Currently when you merge a pull request in Stash, the resulting commit message for the merge includes the messages from all the individual commits being merged. This causes us problems because we use Crucible to generate release reports, and merging from our release branch into develop (for hot fixes added to release after cutting the branch off develop) causes JIRAs from previous releases to be included in the report for the next release.

It would be great to have an option to NOT include the individual commit messages in the merge commit.