add key to FileInfo

MatchingKey is not the thing to use when matching on actual worktreee
files.

Fix reversion in 8.20201116 that made include= and exclude= in
preferred/required content expressions match a path relative to the current
directory, rather than the path from the top of the repository.

Annex/Import.hs

@@ -531,6 +531,7 @@ importKeys remote importtreeconfig importcontent importablecontents = do
 			let mi = MatchingFile FileInfo
 				{ matchFile = f
 				, contentFile = Just tmpfile
+				, matchKey = Nothing
 				}
 			islargefile <- checkMatcher' matcher mi mempty
 			if islargefile