gofmt clobbers redo buffer

[mkantor]

Steps to reproduce:

1. printf '%s\n' 'package main' 'func foo(){}' > example.go
2. atom example.go
3. Make an edit to the file.
4. Undo your edit.
5. Save the file or otherwise trigger gofmt.
6. Try to redo your edit.

Since gofmt changed the file contents, your redo buffer is gone. ☹️