When a new repo is being added, whether manually or via an incoming Intent,
check the address and fingerprint against repos in the DB. If the repo is
not in the DB, offer to add it. If the repo address is in the DB, then do
more checks:
* If that address has no fingerprint in the DB, then offer to add the new
repo including that fingerprint. This might happen when upgrading a repo
from unsigned to signed.
* if the incoming info matches a repo in the DB, offer to enable that repo
* if the address matches a repo in the DB but the incoming fingerprint does
not match the fingerprint in the DB, warn the user, and tell them to
delete the existing repo if they truly want to override the existing info
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