dvc get --rev doesn't work if master is not a dvc repo

[jackspitts]

Bug Report

Please provide information about your setup

Output of dvc version:

DVC version: 1.1.7
Python version: 3.7.2
Platform: Darwin-18.7.0-x86_64-i386-64bit
Binary: False
Package: pip
Supported remotes: http, https, s3
Cache: reflink - supported, hardlink - supported, symlink - supported
Repo: dvc, git

Additional Information (if any):

Discord link with all info

https://discordapp.com/channels/485586884165107732/485596304961962003/731227119954427904

[efiop]

For the record: the issue is that in external repo we operate on virtual trees(we don't checkout to the branch), but we still read config through regular open() calls, which is causing this issue. Need to make our config use repo's self.tree to read/write configs.