Makes repo existance check more sensitive.

[hackergrrl]

This makes the does-repo-exist check return true if 'config' exists OR
'datastore' exists, rather than a negated AND. This is helpful for js-ipfs or
other implementations which may not use a 'datastore' directory, since it
prevents false positives that could result in go-ipfs ovewriting another ipfs
implementation's repo.

[hackergrrl]

(We really should have a canonical "is this an IPFS repo" check across impls in a spec. cc @diasdavid)

[whyrusleeping]

I'm not sure i agree with this. If a config does not exist, we don't have a repo, regardless of whether or not we have a datastore directory (or a blocks directory, which seems to be missing from this outdated check)

[hackergrrl]

@whyrusleeping: ping. Are we good on this change? (Before I bother rebasing)

[whyrusleeping]

Yeah, i'm okay with this change.

[hackergrrl]

woo hoo -- ready for merge

[hackergrrl]

Anything else we need here?