Borg uses an empty ContentIdentifier for everything; it does not need to
record anything. But that empty value gets stored in the log for each key
that is stored in borg. This unncessarily bloats the size of the git-annex
branch, by one content identifier per key stored in borg.

I think that it also slows down importing many archives from borg,
because for each of them it has to record the content identifier,
which is always the same, but still results in a db write.

Omitting storing any ContentIdentifier would break code such as
Remote.Helper.ExportImport's retrieveKeyFileFromImport.

If the borg Remote could indicate with a flag that it does not use
ContentIdentifiers, then code like that could pass it a null
ContentIdentifier without needing to read it from the db.

Annex.Import uses getContentIdentifierKeys, but only when it's not
thirdpartypopulated. So this change would not break that for borg,
but a clean way to handle that would be to make it also return a null
ContentIdentifier when the remote has the flag set. --[[Joey]]
