As it has Merge in the name and can do diffs and merge resolution, I would have expected the option to take 3 arbitrary files and open a 3+ way merge on them. EG I have a file in a current repo; I want to specify another file that is the common base and a third file that will be the staged changes and these are all separate.
This may not be simple because you’re likely relying on git’s diff based storage, but in some cases I have a repo that has some files where a base version is not in the history at all and I have another change I’d like to merge in, from the other repo that does have the base in the history and some changes not reflected in this repo. But I would normally thing the simplest way for me to do it is to check out the base copy the file to .base., check out the changes I want to bring in and copy them to .new. then open a 3-way merge with . .base. and .new.. Because, to be honest, committing the base in a branch, updating that with a commit of the existing version and then changing that to the new version to view it provides me only one 2 way diff at a time; without a forced merge conflict I’m not going to see how both versions differ from the base and how to merge their intents together.