raco: fix command name selection by prefix

Repairs a problem with 409321c03b.

Merge to v6.9

pkgs/racket-test-extra/tests/raco/prefix.rkt

@@ -0,0 +1,15 @@
+#lang racket/base
+(require compiler/find-exe
+         racket/system)
+
+;; Make sure that `raco` recognizes unambiguous command prefixes.
+;; This test assumes that "pk" will unambiguously refer to "pkg".
+
+(define o (open-output-bytes))
+(parameterize ([current-output-port o])
+  (system* (find-exe)
+           "-N" "raco"
+           "-l-" "raco"
+           "pk"))
+(unless (regexp-match? #rx"raco pkg install" (get-output-bytes o))
+  (error "expected output from `raco pkg`"))

racket/collects/raco/raco.rkt

@@ -52,7 +52,7 @@
        [tool (and (pair? cmdline)
                   (or (hash-ref tools (car cmdline) #f)
                       (and (= (hash-count prefix-tools) 1)
-                           (hash-ref tools (car cmdline) #f))))]
+                           (car (hash-values prefix-tools)))))]
        [ambiguous? (> (hash-count prefix-tools) 1)]
        [show-all?
         (cond