remote: local: support relative paths

[efiop]

E.g. dvc remote add mylocal ../../mydir. Note that same as with cache.dir, path should be resolved relative to config file location, which is .dvc.

[dlazerka]

Great, I would take a caution about going ../ from the root dir (i.e. outside project), might be a security issue. Although I heard that it might be hard to really determine if you're "outside" (symlinks and whatnot).

[efiop]

@dlazerka DVC doesn't have any additional privileges, so it can't do much harm :) Unless I'm missing something.

[dmpetrov]

@dlazerka thank you for your feedback!

A relative path should not base on .dvc. However, we can resolve the path and save it as a relative to .dvc.

#1284 is not a relevant explanation for this case since dvc remote is not a direct interface for config file manipulation. We should support "principle of least surprise" for high-level commands such as dvc remote.

[efiop]

@dmpetrov Turns out git remote add myremote ../somedir writes ../somedir as is to the config file and then resolves it relative to the project's root. So following that logic and considering that cache.dir from #1284 is a shorthand for an implicit remote, we should indeed resolve cache.dir relative to the project's root, as you've mentioned in #1284 Funny enough, when you specify a remote with a relative path in your global(or system) config, git still resolves its path relative to the current project's root, which is extremely odd. That is why I think that resolving based on a config location is much more logical and with the "least surprise", since it provides a unified experience across all commands(config/remote) and all levels of config(local, project, global, system).