refactor: don't litter the source tree with backup turdlets.
refactor: don't litter the source tree with backup turdlets.

Also, restore from backup if the replacement file cannot be written.

LGTM=sameer
R=sameer
CC=golang-codereviews
https://codereview.appspot.com/155660043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools