api/get: local repo with custom rev is not supported

[skshetry]

import dvc.api

dvc.api.get_url("file", repo=".", rev="HEAD")

throws following error:

AssertionError: Custom revision is not supported for local repo

Similarly, issue exists with dvc get --show-url (introduced by #3156):

➜ dvc get . foo --rev=HEAD@{5.days.ago} --show-url
ERROR: unexpected error - Custom revision is not supported for local repo

[Suor]

Title is misleading dvc get will work with rev, while dvc get --show-url won't because it's a completely different command. Call it dvc get-url maybe?

[skshetry]

We already have a get-url: https://dvc.org/doc/command-reference/get-url
And, for now, --show-url is a part of get. It'd be good to have a better category (but should be covered by description). 🙂