Allow filter to be empty for aptly * search commands

Empty filter implies "select all packages".

cmd/repo_search.go

@@ -8,11 +8,13 @@ import (
 func makeCmdRepoSearch() *commander.Command {
 	cmd := &commander.Command{
 		Run:       aptlySnapshotMirrorRepoSearch,
-		UsageLine: "search <name> <package-query>",
+		UsageLine: "search <name> [<package-query>]",
 		Short:     "search repo for packages matching query",
 		Long: `
 Command search displays list of packages in local repository that match package query
 
+If query is not specified, all the packages are displayed.
+
 Example:
 
     $ aptly repo search my-software '$Architecture (i386), Name (% *-dev)'