Steps to reproduce

1. Create a branch 'production'
2. Perform a commit in the branch
3. Run the following command in the repo so that it will have ambiguous refname:

   cp .git/ref/heads/production .git/production
   

4. Restart SourceTree
5. Checkout master branch
6. Make sure that the option "Always display full console output" is disabled
7. Merge production branch into master branch
   The merge fail but there isn't any error in SourceTree
8. Enable the option "Always display full console output"
9. Merge production branch into master branch

   git -c diff.mnemonicprefix=false -c core.quotepath=false -c credential.helper=sourcetree merge production 
   Already up-to-date.
   warning: refname 'production' is ambiguous.
   Completed successfully
   

SourceTree should have shown that the merge have failed and display the error.