How to reproduce:
1. Download the provided "PreMerge" and "Master" projects
2. Make a repository on GitHub
3. Push the "Master" project on its own branch
4. Push the "PreMerge" project up its own branch
5. Clone the repository using Sourcetree
6. Push "Master" branch into "PreMerge" branch
7. Inspect the merge conflicts
8. Try resolving the conflicts by using Resolve Conflicts->Launch External Merge Tool (Merge tool can be located at <Path to Unity>/Unity.app/Contents/Tools/UnityYAMLMerge)
9. Inspect the results

Expected result: Conflicts are resolved
Actual result: Conflicts are not resolved and the merge fails silently with a prompt shown briefly

Reproducible with: 2019.4.24f1

Documentation on how to set up Smart Merge on Sourcetree: https://docs.unity3d.com/Manual/SmartMerge.html