If a version-controlled file is ignored by git, and a conflict arises
on the file, and I use mergetool to resolve the conflict, then
mergetool fails with a message like:

The following paths are ignored by one of your .gitignore files:
a
Use -f if you really want to add them.

The problem disappears if I edit the git-mergetool script to always
pass -f to "git add", but I'm not sure if that's the right fix; I have
a vague feeling that "git-update-index --add" could be more correct.

I have attached a script that reproduces the problem.