FR: `merge-tools` cannot express "this file should be deleted"

[9999years]

Is your feature request related to a problem? Please describe.
I'd like to use a merge tool to resolve conflicts caused by files being renamed upstream. The resolution I want is "the conflicted file doesn't exist anymore, so we'll delete it entirely". The merge-tools feature doesn't seem able to express this; leaving the output file empty is interpreted as an error:

Error: Failed to resolve conflicts
Caused by: The output file is either unchanged or empty after the editor quit (run with --debug to see the exact invocation).

Describe the solution you'd like
Some sort of API to indicate that the file should be deleted.

Additional context
This sort of conflict (a rename upstream) is tricky to resolve with the current API. Perhaps we could pass the merge tool a blank output directory and instruct it to make edits there, and apply all of those? But that might cause conflicts with other changed files. There's an intersection with #47 more broadly here.

[martinvonz]

This is also something we want so you can open all the files to merge at once if the merge tool supports it. Kinetic Merge does since sageserpent-open/kineticMerge#136. They are waiting on us to add support for it.