You actually CAN use idea as a standalone merge tool[1], I have these lines in my .gitconfig for years now:

   [mergetool "idea"]
    cmd = /usr/bin/env idea merge "$LOCAL" "$REMOTE" "$BASE" "$MERGED"

Then you just run `git mergetool -t idea` and voilà

[1]: https://www.jetbrains.com/help/idea/command-line-merge-tool....