Allow single-letter aliases

Fixes #1074

main/src/main/scala/sbt/Cross.scala

@@ -69,7 +69,7 @@ object Cross {
     token(CrossCommand <~ OptSpace) flatMap { _ =>
       (token(Parser.opt("-v" <~ Space)) ~ token(matched(state.combinedParser))).map {
         case (verbose, command) => CrossArgs(command, verbose.isDefined)
-      } & spacedFirst(CrossCommand)
+      }
     }
 
   private def crossRestoreSessionParser: Parser[String] = token(CrossRestoreSessionCommand)

sbt/src/sbt-test/actions/add-alias/build.sbt

@@ -1,2 +1,3 @@
 addCommandAlias("demo-success", "run true")
 addCommandAlias("demo-failure", "run false")
+addCommandAlias("z", "scalaVersion")

sbt/src/sbt-test/actions/add-alias/test

@@ -1,2 +1,3 @@
 > demo-success
 -> demo-failure
+> +z