git remove merge commit from history

Starting with the repo in the original state

To remove the merge commit and squash the branch into a single commit in the mainline

Use these commands (replacing 5 and 1 with the SHAs of the corresponding commits):

git checkout 5
git reset --soft 1
git commit --amend -m '1 2 3 4 5'
git rebase HEAD master

To retain a merge commit but squash the branch commits into one:

Use these commands (replacing 5, 1 and C with the SHAs of the corresponding commits):

git checkout -b tempbranch 5
git reset --soft 1
git commit --amend -m '1 2 3 4 5'
git checkout C
git merge --no-ff tempbranch
git rebase HEAD master

To remove the merge commit and replace it with individual commits from the branch

Just do (replacing 5 with the SHA of the corresponding commit):

git rebase 5 master

And finally, to remove the branch entirely

Use this command (replacing C and D with the SHAs of the corresponding commits):

git rebase --onto C D~ master

Do git rebase -i <sha before the branches diverged> this will allow you to remove the merge commit and the log will be one single line as you wanted. You can also delete any commits that you do not want any more. The reason that your rebase wasn't working was that you weren't going back far enough.

WARNING: You are rewriting history doing this. Doing this with changes that have been pushed to a remote repo will cause issues. I recommend only doing this with commits that are local.