Add support for UE4Editor merge resolution workflow

[mastercoms]

Git repo now configured properly for UE4Editor to detect a conflict and prompt for a resolution. However, we block launches from happening when a merge conflict is encountered. Thus, we should investigate the UE4Editor workflow for this, and confirm everything is working smoothly. It may require creating a custom mergetool for git (to replace the default p4merge.exe in UE4Editor, which isn't found in path).

[mastercoms]

Partially fixed in fc39d27. Still need to unblock launches when a merge conflict is present.