bootstrap: show available paths help text for aliased subcommands

Running `./x.py build -h -v` shows a list of available build targets,
but the short alias `./x.py b -h -v` does not. Fix so that the aliases
behave the same as their spelled out counterparts.

src/bootstrap/builder.rs

@@ -621,9 +621,9 @@ macro_rules! describe {
 
     pub fn get_help(build: &Build, subcommand: &str) -> Option<String> {
         let kind = match subcommand {
-            "build" => Kind::Build,
-            "doc" => Kind::Doc,
-            "test" => Kind::Test,
+            "build" | "b" => Kind::Build,
+            "doc" | "d" => Kind::Doc,
+            "test" | "t" => Kind::Test,
             "bench" => Kind::Bench,
             "dist" => Kind::Dist,
             "install" => Kind::Install,