Add option to skip "Reset approvals when source branch is modified" if no changes are introduced

When the "Reset approvals when source branch is modified" merge check is enabled, approvals are reset for all updates to the branch including merge commits from master.

When the actual diff in the PR hasn't changed, it'd be nice to avoid the need for everyone to re-approve the PR.

Since merge commits are highlighted separately in commit activity streams we know the updates we want to ignore are already distinguishable in bitbucket's ui; so presumably it could be done

I can understand some projects may not want to ignore those commits, hence suggesting it as an option.

We have one project that auto-updates branches and the reset merge check had to be turned off again, which is a pity.