You don't want
git revert. That undoes a previous commit. You want
git checkout to get git's version of the file from master.
git checkout -- filename.txt
In general, when you want to perform a git operation on a single file, use
Git introduced a new command
git restore in version
2.23.0. Therefore, if you have git version
2.23.0+, you can simply
git restore filename.txt - which does the same thing as
git checkout -- filename.txt. The docs for this command do note that it is currently experimental.