[JENKINS-40381] PreBuildMerge --ff-only fails to send notification mails for non-ff branches

Consider a job is configured with

• Branches to build: Branch specifier: for-master/**
• Additional Behaviours
  • Merge before build
    • Branch to merge to: master
    • Fast-forward mode: --ff-only
  • Calculate changelog against a specific branch
    • Name of branch: master

It is clearly possible, with this configuration, to calculate the changelog and thus identify who should receive failure notifications, even if the branch to build is not a fast-forward.

This isn't done, though, so if someone pushes a branch for-master/foo that is not a fast-forward of master, then the job will fail with a message like this:

ERROR: Branch not suitable for integration as it does not merge cleanly: Command "git merge --ff-only c33c4e240696ae203c257991ef4bfde9a939a4fa" returned status code 128:
stdout: 
stderr: fatal: Not possible to fast-forward, aborting.
...
...
An attempt to send an e-mail to empty list of recipients, ignored.
Finished: FAILURE

The changelog should be computed before the branch merging so a notification can be sent out.

Consider a job is configured with


	
Branches to build: Branch specifier: for-master/**
	
Additional Behaviours
	

		
Merge before build
		

			
Branch to merge to: master
			
Fast-forward mode: --ff-only
		
		
		
Calculate changelog against a specific branch
		

			
Name of branch: master
		
		
	
	



It is clearly possible, with this configuration, to calculate the changelog and thus identify who should receive failure notifications, even if the branch to build is not a fast-forward.

This isn't done, though, so if someone pushes a branch for-master/foo that is not a fast-forward of master, then the job will fail with a message like this:



ERROR: Branch not suitable for integration as it does not merge cleanly: Command "git merge --ff-only c33c4e240696ae203c257991ef4bfde9a939a4fa" returned status code 128:
stdout: 
stderr: fatal: Not possible to fast-forward, aborting.
...
...
An attempt to send an e-mail to empty list of recipients, ignored.
Finished: FAILURE


The changelog should be computed before the branch merging so a notification can be sent out.

Reproduce the Jenkins job configuration with Merge before build, --ff-only, and changelog calculation against master, then trace the merge, changelog, and failure-notification flow in the git-plugin. Done means a non-fast-forward failure still computes the changelog and sends notifications to the identified recipients.