I love `git commit --fixup`

gistfile1.txt

# Have origin be your fork, upstream is upstream.
$ git remote -v
origin	[email protected]:jbeda/kubernetes.git (fetch)
origin	[email protected]:jbeda/kubernetes.git (push)
upstream	[email protected]:kubernetes/kubernetes.git (fetch)
upstream	[email protected]:kubernetes/kubernetes.git (push)

# Make some changes to a bunch of files in a multi commit PR

# Find the commit you want to tweak, copy the hash
$ git log

# Commit that file with the `--fixup` flag.
$ git add file.go
$ git commit --fixup f6d3505ddc6c3944b354cf3f4f2583e6c8775581

# Now rebase (without a fetch) to automatically squash the fixup commits
$ git rebase -i --autosquash upstream/master