Mark issues as fixed only when PR has been merged (BB-14687)

I set up a workflow with a "primary" branch on which only a set of developers are allowed to push. Those who are not, create PR's with changes which are later review and merged/discarded accordingly.

Consider a developer fixing issues #1 and #2 with a PR #3. With the current "Issues" service, issues are marked as fixed as soon as the Fixes BCLOUD-1 is part of the commit message. This means that issue #1 will be marked as fixed even if the pr BCLOUD-3 might not be merged.

As a workaround, the Fixes BCLOUD-1,#2 can be part of the PR commit message (and not referenced in other commits), but then the history of the commit that fixed the issue is lost. I would want the issue to be marked as resolved only when the commit is part of the "primary" branch.

Workflow example:

1. Two issues #1 and #2 are defined.
2. Developer checkouts to a branch: bugfixes.
3. Developer commits a fix to #1: git commit -m "Fix BCLOUD-1" (Issue is not yet marked as solved)
4. Developer commits a fix for #2: git commit -m "Fix BCLOUD-2" (Issue is not yet marked as solved)
5. Developer creates a PR for bugfixes.
6. Contributor merges bugfixes into master
7. Issue service marks #1 and #2 as fixed based on the commits merged into master

A solution for this would be to add an option to the Issue service to restrict from which branches the issues can be marked as fixed. What are your thoughts on this?