[darc/maestro++] Support removing repo merge policies

[mmitche]

@dougbu commented on Wed Jul 03 2019

Currently the only darc commands related to repository policies are get-repository-policies and set-repository-policies. While that covers CRU because set-repository-policies both creates and updates (well, overwrites) policies, there is no command to delete repo policies.

For example, we have old policies for non-existent branches that won't be serviced. They just add noise to darc get-repository-policies --all output. I know --all isn't the default but it's very useful when creating policies for branches that don't yet exist.

For example

darc get-repository-policies --repo https://github.com/aspnet/Extensions --all

https://github.com/aspnet/Extensions @ master
- Merge Policies:
  Standard
https://github.com/aspnet/Extensions @ release/3.0-preview3
- Merge Policies:
  Standard
https://github.com/aspnet/Extensions @ release/3.0-preview4
- Merge Policies:
  Standard
https://github.com/aspnet/Extensions @ release/3.0-preview5
- Merge Policies:
  Standard
https://github.com/aspnet/Extensions @ release/3.0-preview53
- Merge Policies:
  Standard
https://github.com/aspnet/Extensions @ release/3.0-preview6
- Merge Policies:
  Standard
https://github.com/aspnet/Extensions @ release/3.0-preview7
- Merge Policies:
  Standard

Hmm, that output includes policies for a branch that never existed. The 'release/3.0-preview53' repo policies must have been accidentally created (probably by me ☹️).