[git] Git error on commit after merge - fatal: cannot do a partial commit during a merge

git commit -i -m 'merge message' didn't work for me. It said:

fatal: No paths with --include/--only does not make sense.

FWIW, I got here via this related question because I was getting this message:

fatal: You have not concluded your merge (MERGE_HEAD exists).

I also tried mergetool, which said No files need merging. Very confusing! So the MERGE_HEAD is not in a file that needs merging-??

Finally, I used this trick to add only the modified files (did not want to add all the files in my tree, since I have some I want to keep untracked):

git ls-files -m | xargs git add

Then I was finally (!) able to commit and push up. It sure would be nice if git gave you better hints about what to do in these situations.