Didn't like the complicated stuff that happened on credentials:edit. It
would append to .gitattributes multiple times. Though I see why it was
written that way.

I'm cutting off for now, but since this new flow would require each developer
to run --enable perhaps this should really be:

1. Developer enrolls Rails app by running `credentials:diff --enable`
2. credentials:edit checks .gitattributes for `diff=rails_credentials` and
   if the current file is covered by that.
3. If so, set up the "rails_credentials" driver automatically.

Helpers is more for sharing between commands. Since `Diffing` is only
for credentials we should just keep it only for credentials.

Replaces "pretty" with diffing since the former is ambiguous, while
diffing captures what it does. `opt_in` seemed clunky so it's swapped
for the one-word enable.

Address `DEPRECATED: Use assert_nil if expecting nil`

```ruby
$ cd activerecord
$ bin/test test/cases/dirty_test.rb:494
... snip ...
DEPRECATED: Use assert_nil if expecting nil from /home/yahonda/git/rails/activerecord/test/cases/dirty_test.rb:494. This will fail in Minitest 6.
DEPRECATED: Use assert_nil if expecting nil from /home/yahonda/git/rails/activerecord/test/cases/dirty_test.rb:511. This will fail in Minitest 6.
.

Finished in 0.061593s, 16.2356 runs/s, 795.5428 assertions/s.
1 runs, 49 assertions, 0 failures, 0 errors, 0 skips
$
```

Refer seattlerb/minitest#666 rails/rails#27712

typo in word compatibily -> compatibility [ci skip]

Usually the application requires the entire active support at load time
but the configuration happens before it is loaded. For that reason we
need to require the core_ext that we want to use in this file.

Add ability to unset preventing writes

Previously if an app attempts to do a write inside a read request it will be
impossilbe to switch back to writing to the primary. This PR adds an
argument to the `while_preventing_writes` so that we can make sure to
turn it off if we're doing a write on a primary.

Fixes #36830
Co-authored-by: NJohn Crepezzi <john.crepezzi@gmail.com>

Introduce InvalidConfigurationError

This reverts commit dd779c96.

This reverts commit 98d0f7eb.

This reverts commit 8d2866bb.

Remove duplicate method definition

no_color! is already defined in [`Rails::Command::Behavior`][behavior],
which gets [included into `Rails:Generators`][include]. This duplication
came about in 6813edc7 when we introduced the Thor command structure.

We have [test coverage][] to ensure this method still behaves correctly
on `Rails:Generators`.

[behavior]: https://github.com/rails/rails/blob/f7e91c7224560fbd3e95c238a0e8bb362799bcb7/railties/lib/rails/command/behavior.rb#L12-L14
[include]: https://github.com/rails/rails/blob/f7e91c7224560fbd3e95c238a0e8bb362799bcb7/railties/lib/rails/generators.rb#L18
[test coverage]: https://github.com/rails/rails/blob/f7e91c7224560fbd3e95c238a0e8bb362799bcb7/railties/test/generators_test.rb#L163-L168

To avoid "uninitialized constant ActiveSupport::Testing (NameError)"

Add tests for selecting aggregrates

This hack prevails everywhere in the codebase by being copy & pasted, and it's actually not a negative thing but a necessary thing for framework implementors,
so it should better have a name and be a thing.

And with this commit, activesupport/test/abstract_unit.rb now doesn't silently autoload AS::TestCase,
so we're ready to establish clearner environment for running AS tests (probably in later commits)

It's used everywhere, clean and mature enough

In our app at work we had a faked config like this:

```
{ "foo" => :bar, "bar" => { "adapter" => "memory" } }
```

This config is invalid. You can't say for foo env just have a symbol,
nor would this work if you had fa foo env with just a string. A
configuration must be a url or an adapter or a database. Otherwise it's
invalid and we can't parse it.

When this was just yaml turned into hashes you could get away with
passing whatever. It wouldn't work but it wouldn't blow up either.

Now that we're using objects we were returning `nil` for these but that
just means we either blow up on `for_current_env` or compact the
`nil`'s.

I think it's a better user experience to not build the configs and raise
an appropriate error.

This is also an invalid config because if you do pass a string here it
should be a URL.

```
{ "foo" => "bar", "bar" => { "adapter" => "memory" } }
```

Deduplicate joins values

#36805 have one possible regression that failing deduplication if
`joins_values` have complex order (e.g. `joins_values = [join_node_a,
:comments, :tags, join_node_a]`).

This fixes the deduplication to take it in the first phase before
grouping.

These tests verifies that aggregates like `AVG` can be selected as
"virtual attributes" on Active Record models and have the correct
column type.

Added missing comment notation for the example of form_with in form_helpers.md documentation. [ci skip]