Currently, `app:update` generates all contents regardless of the
component using in application.

For example, even if not using Action Cable, `app:update` will generate
a contents related to Action Cable. This is a little inconvenient.
This PR checks the existence of the component and does not generate
unnecessary contents.
Can not check all options in this way. However, it will be able to
prevent the generation of unnecessary files.

By making the Rails minitest behave like a standard minitest plugin
we're much more likely to not break when people use other minitest
plugins. Like minitest-focus and pride.

To do this, we need to behave like minitest: require files up front
and then perform the plugin behavior via the at_exit hook.
This also saves us a fair bit of wrangling with test file loading.

Finally, since the environment and warnings options have to be applied
as early as possible, and since minitest loads plugins at_exit, they
have to be moved to the test command.

* Don't expect the root method.

It's likely this worked because we eagerly loaded the Rails minitest plugin
and that somehow defined a root method on `Rails`.

* Assign a backtrace to failed exceptions.

Otherwise Minitest pukes when attempting to filter the backtrace (which
Rails' backtrace cleaner then removes).

Means the exception message test has to be revised too.

This is likely caused by the rails minitest plugin now being loaded for
these tests and assigning a default backtrace cleaner.

Currently the environment file is not loaded in `dbconsole` command.
Therefore, for example, if use encrypted secrets values in database.yml,
`read_encrypted_secrets` will not be true, so the value can not be
used correctly.

Fixes #29717

When secrets confirmed with the `secrets:edit` command, `secrets.yml.enc`
will change without updating the secrets.

Therefore, even if only want to check secrets, the difference will come
out. This is a little inconvenient.

In order to solve this problem, added the `secrets:show` command.
If just want to check secrets, no difference will occur use this command.

If system test fails, it creates screenshot under `tmp/screenshots`.
https://github.com/rails/rails/blob/34fe2a4fc778d18b7fe6bdf3629c1481bee789b9/actionpack/lib/action_dispatch/system_testing/test_helpers/screenshot_helper.rb#L45

But currently, screenshot files is not cleared by `tmp:clear` task.
This patch make clears screenshot files with `tmp:clear` task as well
as other tmp files.

The `capify` command has been removed by Capistrano 3 and became to
`cap install`.
Therefore, the `capify!` method has no meaning in Capistrano 3.
I think that should deprecate.

Ref: https://github.com/capistrano/capistrano/commit/492793916acf32ffe1604daec6fd4892c8935018

Fixes #28988

* Remove trailing spaces.
* Add backticks around method and command.
* Fix indentation.

The css selectors on the generated error pages are too broad (for example `text-align: center` on `body` and thus bleed out to the following pages (say, by pressing the back button) when using Turbolinks. This commit namespaces all the selectors.

The `master` changelog refer back to the `5-1-stable` changelog as a
base. No need to duplicate backported entries in the changelogs on
`master`.

This is a follow up to a6d065e3. When using `form_with` you must supply
field ids manually. Since the scaffold generator is using labels we
need to make sure that they are linked up properly.

The "-j" option was added 5 years ago (https://github.com/rails/rails/commit/d9c39c3a)
when we wanted to support prototype-rails and jquery-rails.
Prototype is not as popular and jQuery is not a requirement anymore.
Still the "-j" option can be used to install *any* gem that ends in "-rails".

This "might" open security issues and does not bring great benefits anymore.

If you know which "-rails"-ending gem you want to install, you can manually
add it to the Gemfile just like any other gem.

The entry for PR #25430 is currently present both in the CHANGELOG
for Rails 5.0 and for Rails 5.1:

https://github.com/rails/rails/blame/9d3a352777c2594123583b0bc02d0dd80f1e385b/railties/CHANGELOG.md#L61-L72

Since the PR was backported to 5-0-stable in #25499, I believe it
should be removed from the CHANGELOG of 5.1, otherwise it looks like
something changed from 5.0 to 5.1

Occasionally we update the file generated by engine.
Therefore, I think that there is a task for updating as well as
application in the engine, it is convenient for updating.

Was added in #25248 by @bluesh55

[ci skip]

These tests may be expansive so let's only allow users to run them
through `bin/rails test:system` or by passing a path to the `test`
command.

The same applies for `bin/rake test`.

Refs #28109.

Includes a script to ease an app's upgrade.

Just dispatch to the command help itself for more info.

Replaces the rake_patterns instance variable with simple require, as
`autorun` will run tests from all eagerly required test files.

Fixes #27801

* If `--skip-git` is not specified initalize git repo
 when creating new rails app

Currently, all rails commands can be executed in engine,
but `server`, `console`, `dbconsole` and `runner` do not work.

This make all rails commands work in engine.
Related to #22588

- Fixes #27591.