Rework collection resultion for `ec.rebuild`, in preparation for parallelization. (#7420)

* Rework collection resultion for `ec.rebuild`, in preparation for parallelization.

See https://github.com/seaweedfs/seaweedfs/issues/7416 .

* simplify

* Update weed/shell/command_ec_rebuild.go

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

---------

Co-authored-by: chrislu <chris.lu@gmail.com>
Co-authored-by: Chris Lu <chrislusf@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

weed/shell/command_ec_rebuild.go

@@ -79,23 +79,23 @@ func (c *commandEcRebuild) Do(args []string, commandEnv *CommandEnv, writer io.W
 		return err
 	}
 
+	var collections []string
 	if *collection == "EACH_COLLECTION" {
-		collections, err := ListCollectionNames(commandEnv, false, true)
+		collections, err = ListCollectionNames(commandEnv, false, true)
 		if err != nil {
 			return err
 		}
-		fmt.Printf("rebuildEcVolumes collections %+v\n", len(collections))
+	} else {
+		collections = []string{*collection}
+	}
+
+	fmt.Printf("rebuildEcVolumes for %d collection(s)\n", len(collections))
 	for _, c := range collections {
-			fmt.Printf("rebuildEcVolumes collection %+v\n", c)
+		fmt.Printf("rebuildEcVolumes collection %s\n", c)
 		if err = rebuildEcVolumes(commandEnv, allEcNodes, c, writer, *applyChanges); err != nil {
 			return err
 		}
 	}
-	} else {
-		if err = rebuildEcVolumes(commandEnv, allEcNodes, *collection, writer, *applyChanges); err != nil {
-			return err
-		}
-	}
 
 	return nil
 }