investigate error handlling 'dvc metrics show -a' fetch remote branch

[AlessandroBregoli]

Executing 'dvc metrics show -a` ask for github credentials in loop. If there is an ssh key for github the command work but is really slow (more than a minute). If no remote is set for git all work without problem.

Please provide information about your setup
DVC version 0.50.1, Installed with pip3 on ubuntu 18.04

[efiop]

Discord context https://discordapp.com/channels/485586884165107732/485596304961962003/595218255489138690

[efiop]

culprit https://github.com/iterative/dvc/blob/0.50.1/dvc/scm/git/tree.py#L81

[efiop]

This is where we actually need that branch fetching https://github.com/iterative/dvc/blob/0.50.1/dvc/external_repo.py#L66

[efiop]

Related #2201

[Suor]

Diagnose: show-ref --verify requires exact ref path like refs/heads/some-branch and we use non-qualified branch names got from repo.scm.list_branches().

I suggest to never fetch anything as a side effect. We should always work with working tree and git repo state in its actual state. @efiop do we still need that ._try_fetch_from_remote()?

[efiop]

@Suor Indeed, after #2218 we don't need _try_fetch_from_remote() at all. Thank you for investigating!