If your merge was not too complicated another option would be to:
- Move all the changes including the merge changes back to stash using "git stash"
- Run the merge again and commit your changes (without the changes from the dropped stash)
- Run a "git stash pop" which should ignore all the changes from your previous merge since the files are identical now.
After that you are left with only the changes from the stash you dropped too early.