verify associated files when checking numcopies

Most of this is just refactoring. But, handleDropsFrom
did not verify that associated files from the keys db were still
accurate, and has now been fixed to.

A minor improvement to this would be to avoid calling catKeyFile
twice on the same file, when getting the numcopies and mincopies value,
in the common case where the same file has the highest value for both.
But, it avoids checking every associated file, so it will scale well to
lots of dups already.

Sponsored-by: Kevin Mueller on Patreon

Database/Keys.hs

@@ -14,6 +14,7 @@ module Database.Keys (
 	closeDb,
 	addAssociatedFile,
 	getAssociatedFiles,
+	getAssociatedFilesIncluding,
 	getAssociatedKey,
 	removeAssociatedFile,
 	storeInodeCaches,
@@ -155,6 +156,15 @@ addAssociatedFile k f = runWriterIO $ SQL.addAssociatedFile k f
 getAssociatedFiles :: Key -> Annex [TopFilePath]
 getAssociatedFiles = runReaderIO . SQL.getAssociatedFiles
 
+{- Include a known associated file along with any recorded in the database. -}
+getAssociatedFilesIncluding :: AssociatedFile -> Key -> Annex [RawFilePath]
+getAssociatedFilesIncluding afile k = do
+	g <- Annex.gitRepo
+	l <- map (`fromTopFilePath` g) <$> getAssociatedFiles k
+	return $ case afile of
+		AssociatedFile (Just f) -> f : filter (/= f) l
+		AssociatedFile Nothing -> l
+
 {- Gets any keys that are on record as having a particular associated file.
  - (Should be one or none but the database doesn't enforce that.) -}
 getAssociatedKey :: TopFilePath -> Annex [Key]