Prevent sandbox from using same Github repo:branch as parent

[taylordowns2000]

prevent people from linking a sandbox to its parent repo:branch

[theroinaochieng]

Hi @taylordowns2000 I'm moving this back to the backlog, I can't seem to find the PR for this issue

[taylordowns2000]

Sorry @theroinaochieng , i've linked it properly now. (it was #4722 from May 8th.)