Load a .rb file only once in irb_require

[youdie006]

For a file name ending in .rb, irb_require does if irb_load(file_name) (lib/irb/command/load.rb:54). irb_load returns the value of eval_input, which is nil, so the file is evaluated but never pushed to $", and execution falls through to irb_load(file_name + ".rb") (LoadError) and then Kernel#require, which evaluates it again.

With a file that does $count += 1 on the load path:

irb> $count = 0
irb> irb_require 'probe.rb'
irb> $count   # master: 2, this PR: 1

This calls irb_load without the check, like the no-extension branch below it. A file that is not on the load path still raises LoadError inside the begin and falls back to Kernel#require as before.

The new test sits between the other irb_require tests in test/irb/test_command.rb and fails on master. test/irb/test_command.rb passes on Ruby 3.4 (57 tests), the irb_require/irb_load tests on 2.7, and rubocop reports no offenses on the changed files; I did not run the full rake test matrix.

Written with AI assistance (Claude); I have reviewed the change.