You can fetch and then check out only one file in this way:
git fetch
git checkout -m <revision> <yourfilepath>
git add <yourfilepath>
git commit
Regarding the git checkout
command:
<revision>
-- a branch name, i.e. origin/master
<yourfilepath>
does not include the repository name (that you can get from clicking copy path
button on a file page on GitHub), i.e. README.md