sh: flush stdout before executing the shell

Closes Homebrew/homebrew#48268.

Signed-off-by: Baptiste Fontaine <batifon@yahoo.fr>

Library/Homebrew/cmd/sh.rb

@@ -24,6 +24,7 @@ module Homebrew
          ignore our configuration.
          When done, type `exit'.
          EOS
+    $stdout.flush
     exec ENV["SHELL"]
   end
 end

Library/Homebrew/test/test_integration_cmds.rb

@@ -284,6 +284,11 @@ class IntegrationCommandTests < Homebrew::TestCase
     (HOMEBREW_REPOSITORY/".git").unlink
   end
 
+  def test_sh
+    assert_match "Your shell has been configured",
+                 cmd("sh", {"SHELL" => "/usr/bin/true"})
+  end
+
   def test_custom_command
     mktmpdir do |path|
       cmd = "int-test-#{rand}"