git show somebranch:path/to/your/file
you can also do multiple files and have them concatenated:
git show branchA~10:fileA branchB^^:fileB
You do not have to provide the full path to the file, relative paths are acceptable e.g.:
git show branchA~10:../src/hello.c
If you want to get the file in the local directory (revert just one file) you can checkout:
git checkout somebranch^^^ -- path/to/file