Don't require a lock for the dvc root command

[villasv]

Is there any way that the dvc root would change in the middle of some dvc command that acquired lock? If not, I think dvc root could simply skip acquiring a lock so it can be used freely.

[efiop]

Good point! Could you please also share how you use dvc root in parallel with other dvc commands, so we are on the same page?

[villasv]

I ran into this because I've been experimenting with self-reporting data pipelines, and that diff script I mentioned in #770 plays a role on this. At first, it couldn't be used inside a dvc run because it will issue a dvc root as subprocess.

I've worked around that using git rev-parse --show-toplevel, but that only works because I assume the git root and dvc root to be the same.

[efiop]

Got it. Thank you! Patch is going to be ready for merge in ~10 minutes or so.