Add branches attribute to github_repository

[mikeroll]

Addresses #748.

Adding branches to both the resource and the data source.

[jcudit]

Can we get a test added or modified to incorporate the new field? Otherwise looks 👌🏾 with the docs update and implementation!

[mikeroll]

@jcudit I've only seen tests around for more advanced stuff like resource modification and nontrivial logic, so I figured we don't normally test simple things like this. I'll have another look though!

[jcudit]

Looks good after verifying locally:

STATE:

data.github_repository.test:
  ID = tf-acc-hgsf1
  provider = provider.github
  allow_merge_commit = true
  allow_rebase_merge = true
  allow_squash_merge = true
  archived = false
  branches.# = 1
  branches.0.name = main
  branches.0.protected = false

[mikeroll]

Thank you for finalising this, I got distracted by other things unfortunately.

[tobiassjosten]

Is this released? It's in the documentation but I'm getting an error when trying to use it just now (Terraform v1.0.3 and integrations/terraform-provider-github v4.17.0):

╷
│ Error: Unsupported attribute
│
│   on asdf.tf line 41, in resource "github_branch_default" "asdf":
│   41:   count      = min(1, length(github_repository.asdf.branches))
│
│ This object has no argument, nested block, or exported attribute named "branches".
╵