drop, move, mirror: when two files have the same content, honor the max numcopies and requiredcopies

Eg, before with a .gitattributes like:

*.2 annex.numcopies=2
*.1 annex.numcopies=1

And foo.1 and foo.2 having the same content and key, git-annex drop foo.1 foo.2
would succeed, leaving just 1 copy, despite foo.2 needing 2 copies.
It dropped foo.1 first and then skipped foo.2 since its content was gone.

Now that the keys database includes locked files, this longstanding wart
can be fixed.

Sponsored-by: Noam Kremen on Patreon

Command/Drop.hs

@@ -227,7 +227,7 @@ checkRequiredContent (PreferredContentChecked False) u k afile =
  - copies on other semitrusted repositories. -}
 checkDropAuto :: Bool -> Maybe Remote -> AssociatedFile -> Key -> (NumCopies -> MinCopies -> CommandStart) -> CommandStart
 checkDropAuto automode mremote afile key a =
-	go =<< getAssociatedFileNumMinCopies afile
+	go =<< getSafestNumMinCopies afile key
   where
 	go (numcopies, mincopies)
 		| automode = do