Added LinkType to ProvidedInfo, and unified MatchingKey with
ProvidedInfo. They're both used in the same way, so there was no real
reason to keep separate.
Note that addLocked and addUnlocked still set matchNeedsFileName,
because to handle MatchingFile, they do need it. However, they
don't use it when MatchingInfo is provided. This should be ok,
the --branch case will be able skip checking matchNeedsFileName,
since it will provide a filename in any case.
