daniellansun/how-to-squash-commits-in-git.md

This method avoids merge conflicts if you have periodically pulled master into your branch. It also gives you the opportunity to squash into more than 1 commit, or to re-arrange your code into completely different commits (e.g. if you ended up working on three different features but the commits were not consecutive).

Note: You cannot use this method if you intend to open a pull request to merge your feature branch. This method requires committing directly to master.

Switch to the master branch and make sure you are up to date:

git checkout master && git pull

Merge your feature branch into the master branch locally:

git merge feature_branch

Reset the local master branch to origin's state:

git reset origin/master

Now all of your changes are considered as unstaged changed. You can stage and commit them into one or more commits.

git add . --all
git commit

(Source)

This method only allows you to squash the last X consecutive commits into a single commit. Also, if you have merged master into your branch along the way, you will have to manually merge your new (squashed) commit into master and resolve the merge conflicts.

Use this method if you have not merged master into your branch, you plan to combine all commits into one, and you only changed one feature of the project; or, regardless of those conditions, you must use this method if you intend to open a pull request to merge your code.

To squash the last 3 commits into one:

git reset --soft HEAD~3
git commit -m "New message for the combined commit"

git push origin +name-of-branch

The plus sign forces the remote branch to accept your rewritten history, otherwise you will end up with divergent branches

git push origin name-of-branch

In other words, just a normal push like any other

Main source: http://stackoverflow.com/a/5201642/348995
Source for info about when commits where already pushed: http://stackoverflow.com/a/5668050/348995