git rebase after previous git merge

Rebasing to get a "clean" history is overrated. The best way if you want to preserve history is just to do the merge instead of a rebase. That way if you ever need to go back to a revision, it is exactly the same as the one you tested during development. That also solves your issue about the previously solved merge conflicts.

If you don't care about preserving history, you can create a new branch off of master, check it out, then do a git read-tree -u -m dev to update your working tree to match the dev branch. Then you can commit everything into one big commit and merge it into master as normal.

git merge --squash is now my preferred way of rebasing after a large amount of work and many merges (see this answer). If the branch you're working on is called my-branch and you want to rebase from master then just do the following:

git checkout my-branch
git branch -m my-branch-old
git checkout master
git checkout -b my-branch
git merge --squash my-branch-old
git commit