For people hacking on stuff using github repos:
Whenever you fork someone else's repo, make some changes to it, and then
want to update your repo with the changes made upstream by the author,
it's easy enough as:
# Add the remote, call it "upstream":
git remote add upstream https://github.com/whoever/whatever.git
# Fetch all the branches of that remote into remote-tracking branches,
# such as upstream/master:
git fetch upstream
# Make sure that you're on your master branch:
git checkout master
# Rewrite your master branch so that any commits of yours that
# aren't already in upstream/master are replayed on top of that
# other branch:
git rebase upstream/master