TL;DR: Merge branch with latest master:

git fetch && git merge origin/master

Given branches feature and master:

git branch
* feature
  master

Merging master into your feature branch can be tedious:

git checkout master &&
  git pull &&
  git checkout feature &&
  git merge master

Luckily, there’s a solution without having to checkout another branch:

git fetch &&
  git merge origin/master

This means you can rebase your branch with master:

git fetch &&
  git rebase origin/master

Rebase rewinds and replays your commits on top of HEAD, which requires a force push if you’ve pushed your branch to remote:

git push -f