butler

butler is a command-line tool written in Go
itch (the itch.io app) uses it for some network, filesystem and patching operations:
Usage & rationale
butler's diffing & patching hasn't been officially released yet, but
Amos published a small write-up on the itch community.
Early testers are welcome, especially for the sign, verify, diff, and patch
commands, which are detailed within the program's inline help.
License
Licensed under MIT License, see LICENSE for details.