listRemoteBranches

From #43

Currently, the listBranches function only looks for heads. Could you add a function that lists remote branches (or all branches?).

I think… add a listRemoteBranches function that takes a remote argument that defaults to undefined, and returns all remote branches, or if remote is specified returns just the branches in that remote.

[wmhilton]

added in v0.2.0

[mojavelinux]

I think you could avoid the extra function by allowing the remote property to be specified for listBranches.

const branches = await git.listBranches({
  fs,
  dir,
  remote: 'origin'
})

If you don’t specify a remote, then it returns local branches (as it does now).