install_git fails to install with SHA ref

[MilesMcBain]

When an SHA is given for ref in install_git() with git = "external" it fails with

Error in read.table(file = file, header = header, sep = sep, quote = quote,  : 
  no lines available in input

This seems to be because remote_sha.xgit_remote tries to execute a command equivalent to:

git ls-remote https://github.com/milesmcbain/datapasta.git dddf83b317254c495c38897bf857a9e6a032070f

Which returns no output.

I see this case is handled in remote_sha.git2r_remote with this this code:

    # If none found, it is either a SHA, so return the pinned sha or NA
    if (length(found) == 0) {
      return(remote$ref %||% NA_character_)
    }

And I believe a similar strategy would resolve this issue, e.g.

 refs <- git(paste("ls-remote", url, ref))

    if (length(refs) == 0) {
      return(remote$ref %||% NA_character_)
    }