cli: fix matching of mirror identifiers (fixes #19)

Having two given mirrors:
 - test.example.com
 - secondtest.example.com

Matching for test.example.com wasn't possible because the two
identifiers contained the exact same substring.

commands.go

@@ -573,6 +573,9 @@ func (c *cli) matchMirror(text string) (list []string, err error) {
 	}
 
 	for _, e := range mirrorsIDs {
+		if text == e {
+			return []string{e}, nil
+		}
 		if strings.Contains(e, text) {
 			list = append(list, e)
 		}