dvc get from dvc-branch in not dvc-repo

[dmpetrov]

$ dvc -V
0.70.0+38be14

master branch in https://github.com/dmpetrov/test_repo is NOT a DVC-repo. However, the extracted branch b-test IS DVC-repo and should work:

$ dvc get https://github.com/dmpetrov/test_repo --rev  b-test b-file
ERROR: failed to get 'b-file' from 'https://github.com/dmpetrov/test_repo' - URL 'https://github.com/dmpetrov/test_repo' is not a dvc repository.

This a regression since this command worked before: dvc get https://github.com/topusOctopus/mmdetection/ --rev fasterRCNN_resnet50_support model_weights/faster_rcnn_r50_c4_2x-6e4fdf4f.pth

[pared]

Reproduction script:

#!/bin/bash

rm -rf repo remote_repo storage git_repo
mkdir repo remote_repo storage git_repo

maindir=$(pwd)
pushd remote_repo
git init >> /dev/null 

echo README >> README.md
git add README.md
git commit -m "add readme"

git checkout -b dvc_branch
dvc init -q
echo data >> data
dvc add -q data
dvc remote add -d str $maindir/storage
git add .dvc/config data.dvc .gitignore
dvc push -q

popd
pushd repo

git init >> /dev/null && dvc init -q

set -x
set -e

dvc get $maindir/remote_repo --rev dvc_branch data