Merge editor: provide labels for undo operations to help local history timeline

The local history view will use the label of the undo operation as label for the entry to show a more specific label than just "File Saved": https://github.com/microsoft/vscode/blob/6d7beb43a0a3de03fe2b1ee53899416fac36154c/src/vs/platform/undoRedo/common/undoRedo.ts#L26  I think I heard that some model operations from the merge editor have semantic, e.g. when you accept a conflict from one side or the other. Would be great if the relevant model change could have a semantic label for the undo operation. I even think we may show this label somewhere else in the "Edit" menu.