$ git difftool
error: 'tortoisemerge' can only be used to resolve merges
https://github.com/git-for-windows/git/blob/main/mergetools/tortoisemerge#L2
This file is read as the toolname matches a built-in tool, regardless if the tool is manually configured
https://github.com/git-for-windows/git/blob/main/git-mergetool--lib.sh#L182
The workaround is to create a custom tool (with any other name, 'tortoisemerge' is the only tool known by Git that blocks diff).
Is there any history that blocks tortoisegitmerge (or tortoisemerge) as a difftool that anyone knows about?
This should be patched in Git, but I expect there to be questions from Git maintainers.
/Gerhard