Avoid the --fast option preventing checksumming in some cases it was not supposed to

fsck --fast was intended to disable checksumming, but checksumming is done
after transfers too. Due to the check being in the non-incremental path,
it would only affect non-incremental checksumming during a transfer,
and I'm not 100% sure that it was a problem.

Also, when using an external backend that does checksumming, fsck --fast
didn't disable it and now does.

CHANGELOG

@@ -2,6 +2,8 @@ git-annex (10.20240431) UNRELEASED; urgency=medium
 
   * fsck: Fix recent reversion that made it say it was checksumming files
     whose content is not present.
+  * Avoid the --fast option preventing checksumming in some cases it
+    was not supposed to.
   * Typo fixes.
     Thanks, Yaroslav Halchenko
 

Command/Fsck.hs

@@ -528,7 +528,10 @@ checkBackendRemote key remote ai localcopy =
 	checkBackendOr (badContentRemote remote localcopy) key localcopy ai
 
 checkBackendOr :: (Key -> Annex String) -> Key -> RawFilePath -> ActionItem -> Annex Bool
-checkBackendOr bad key file ai = do
+checkBackendOr bad key file ai =
+	ifM (Annex.getRead Annex.fast)
+		( return True
+		, do
 			ok <- verifyKeyContent' key file
 			unless ok $ do
 				msg <- bad key
@@ -536,6 +539,7 @@ checkBackendOr bad key file ai = do
 					<> ": Bad file content; "
 					<> UnquotedString msg
 			return ok
+		)
 
 {- Check, if there are InodeCaches recorded for a key, that one of them
  - matches the object file. There are situations where the InodeCache