avoid logging location of GIT keys

It's not necessary to log location of GIT keys, because these files are
not annexed files and so git-annex will never need to get them.

This corresponds to code in Annex.Import that already checked before
updating the location log when handling deleted files.

Older versions of git-annex that used SHA1 keys for non-annexed files
also unncessarily updated the location log for them.

GIT keys still appear in the git-annex branch for content identifier
logs, so kept the documentation of them in backends.mdwn

This commit was sponsored by Jake Vosloo on Patreon.

Command/Export.hs

@@ -319,7 +319,9 @@ cleanupExport :: Remote -> ExportHandle -> Key -> ExportLocation -> Bool -> Comm
 cleanupExport r db ek loc sent = do
 	liftIO $ addExportedLocation db ek loc
 	when sent $
-		logChange ek (uuid r) InfoPresent
+		case keyGitSha ek of
+			Nothing -> logChange ek (uuid r) InfoPresent
+			Just _ -> noop
 	return True
 
 startUnexport :: Remote -> ExportHandle -> TopFilePath -> [Git.Sha] -> CommandStart