Squash a single conflict fixing incorrect merge conflict resolution into merge commit

If you just committed your last fix commit that you talked about, you can do this:

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git reset --soft HEAD~ # keep the pigtail
git commit --amend --no-edit

Voilá!

If the merge you want to fixup is at the tip of a branch, you can amend the commit:

1. Make sure the merge commit is the currently checked out commit using git status, git log etc
2. Make your required change and use git add as required
3. Run git commit --amend; if you want to keep the original message, add --no-edit
4. Verify the result with git show HEAD or git log --graph - you should see that the current commit is still listed as a merge of the original two parents

This behaviour is explicitly documented:

The new commit has the same parents and author as the current one (the --reset-author option can countermand this).

If there are other commits after the merge, this is slightly more difficult; however, if there are no other merges after the one you want to fix, you could rewind to just after the merge, then replay the remaining commits on top.

e.g. if your branch is called my-feature and the merge commit to fix is abc123def

1. Create a temporary branch from the merge commit: git switch -c temp-to-fix-merge abc123def
2. Make your changes, and use git commit --amend as above
3. Switch back to your original branch: git switch my-feature
4. Replay commits after the merge onto the new version of the merge: git rebase abc123def --onto temp-to-fix-merge
5. Tidy up the temp branch: git branch -d temp-to-fix-merge

Just completing @IMSoP's answer: here is a way to use git rebase -i and add a "break" instruction to manually "stitch together" the fixup and the merge commit:

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[mybox] (left)$ git log --oneline --graph
* 7e4759e (HEAD -> left) create h.txt
* 9bcd31b create g.txt
* 21d3b5c add fix to merge commit (update e.txt)
*   fc44140 Merge branch 'right' into left
|\  
| * 49d3a5d (right) create f.txt
| * 9d7cc54 create e.txt
* | e5f7da7 create d.txt
* | d8c7457 create c.txt
|/  
* 8be599b create b.txt
* f137b07 create a.txt
* 1e8ac8d first commit

[mybox] (left)$ git rebase -i fc44140   # <- the merge commit

### an editor opens:
#pick 21d3b5c add fix to merge commit (update e.txt)
#pick 9bcd31b create g.txt
#pick 7e4759e create h.txt
#
## Rebase fc44140..7e4759e onto fc44140 (3 commands)
## ...

### add a 'break' line right after the 'add fix' commit:
#pick 21d3b5c add fix to merge commit (update e.txt)
#break   # <- here
#pick 9bcd31b create g.txt
#pick 7e4759e create h.txt
#
### save & exit

Stopped at 21d3b5c (add fix to merge commit (update e.txt))
# ammend the merge commit:
[mybox] (left|REBASE 2/4)$ git reset --soft HEAD~ && git commit --amend --no-edit

# proceed with the rebase:
[mybox] (left|REBASE 2/4)$ git rebase --continue

Unfortunately, no. I don't think it’s possible for you to squash a commit into a merge commit using git standard tool (rebase commit --amend) without redoing the resolution manually. But you can recreate the merge from scratch this time very cleanly using --no-commit. Alternatively, you can avoid this by fixing everything before committing the merge.