If the code which is eventually merged is mostly the contributor's code, and you just had to do some small tweaks to get it up to the project's standard, then c

If the code which is eventually merged is mostly the contributor's code, and you just had to do some small tweaks to get it up to the project's standard, then commit and merge it under their name. If the changes are so major that the code cannot reasonably be considered theirs any more, then it would be dishonest to merge it under their name, and would not be doing them or anyone else a favor. In that case, commit and merge under your own name, but include a note in the commit message thanking them for their contribution.