Feature request: Support checking/listing squash merged branches

[calvn]

Support checking/listing branches that have been squash merged into a particular branch (e.g. master) by extending the git branch --merged command. The native git branch --merged command does not detect branches merged by this method, but many GH projects do squash merging to reduce the number of commits on the repository.

This feature would make it trivial to clean up local branches that have already been squash merged.

[turadg]

Noting this error output to help direct people here who search on it,

warning: `feature' was deleted on origin, but appears not merged into master

I get that on git sync. Spec in
https://github.com/github/hub/blob/4777bd363058213605ebb292b35ef1a6edb3663c/features/sync.feature#L48-L56