Describe the feature or problem you’d like to solve

For now, there is no way to update gist files using input from another program

echo "abc" | gh gist edit -f myfile.txt <gist id>

just runs an editor.

Proposed solutions

1. Detect input from stdin, like gh gist create or
2. Add an another command (gh gist update, gh gist rewrite, etc), with gh gist create-like semantics.

• How will it benefit CLI and its users?
  It's much simpler to update gist from any source directly instead of copying it to editor.