It looks like what you're really looking for is a local branch that you never push to the remote, and 'git diff master...' with 'git rebase'.

Create a local branch, apply the sledgehammer, and start reviewing changes. Use 'git diff master...' to review changes. (This is short for 'git diff master...HEAD' which in turn stands for 'compare current branch's HEAD with the commit on master off of which you've branched.) 'git add -p' and commit changes that you like.

Iterate until you're happy with the result. You will have ended up with a few commits on your local branch. Use 'git rebase -i master' to squash all commit in a single one. Finally, check out the master branch and merge your local branch in, preferably with --ff.