How to undo local changes to a specific file [duplicate]
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 -- filename.
2020 Update
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.