How to discard local commits in Git?

As an aside, apart from the answer by mipadi (which should work by the way), you should know that doing:

git branch -D master
git checkout master

also does exactly what you want without having to redownload everything (your quote paraphrased). That is because your local repo contains a copy of the remote repo (and that copy is not the same as your local directory, it is not even the same as your checked out branch).

Wiping out a branch is perfectly safe and reconstructing that branch is very fast and involves no network traffic. Remember, git is primarily a local repo by design. Even remote branches have a copy on the local. There's only a bit of metadata that tells git that a specific local copy is actually a remote branch. In git, all files are on your hard disk all the time.

If you don't have any branches other than master, you should:

git checkout -b 'temp'
git branch -D master
git checkout master
git branch -D temp

git reset --hard origin/master

will remove all commits not in origin/master where origin is the repo name and master is the name of the branch.