Make "s3sync sync foo bucket:bar" idempotent

Previously, running "s3sync foo bucket:bar" twice in a row, regardless
of whether the contents of "foo" had changed, would result in the second
run uploading and then deleting the same files, leaving you with an empty
S3 bucker:folder.

The other direction, "s3sync sync bucket:bar foo", remains idempotent,
if a little wonky.

Rakefile

@@ -1,3 +1,6 @@
 require "bundler/setup"
 require "bundler/gem_tasks"
 require "bump/tasks"
+
+require 'rspec/core/rake_task'
+RSpec::Core::RakeTask.new(:spec)

lib/s3sync/sync.rb

@@ -314,6 +314,9 @@ def read_tree_remote location
     def read_trees source, destination
       if source.local?
         source_tree = LocalDirectory.new(source.path).list_files
+        source_tree = source_tree.reduce({}) do |a,(_,v)|
+          key = S3Sync.safe_join([destination.path, v.path]); a[key] = v; a
+        end
         destination_tree = read_tree_remote destination
       else
         source_tree = read_tree_remote source