Git: Revert file to master

The same approach applies to any branch, but reverting a file to its state in master seems to come up most often.
Firstly, in case you need to list which files have changed between your active branch and master:

git diff --name-status master

Then to revert the file to its state in master:

git checkout master path/to/file

At this point, the file will already be staged, so if you need to unstage it:

git reset path/to/file