Programmatically get default branch name

[nikolas-burkoff]

At the moment this is hard-coded as "main" but I think we can get the default branch out and pass that as fallback argument to determine_ref

[nikolas-burkoff]

Does this work with graphQL - if so do this

[nikolas-burkoff]

This can be done in github:

repository(owner: "openpharma", name: "stageddeps.garden") {
  defaultBranchRef {
    name
  }
} 

and gitlab:

{
  project(fullPath: "path/to/repo") {
    repository {
      rootRef
    }
  }
}

[nikolas-burkoff]

I can't find a way of doing this with git2r - I think this works:

It adds gert as an imports (but that is already required as we import devtools which depends on usethis which imports gert) - also I think we should move to using gert anyway

(Note usethis has a function git_branch_default which may be helpful)

[nikolas-burkoff]

Closing in favour of https://github.com/openpharma/staged.dependencies/issues/134