[tool] Move changed file detection to base command class

[stuartmorgan-g]

Consolidates the code to find all changed file paths into the PackageLoopingCommand class that is the base of almost all of the repo tooling commands. This in a preparatory PR for a future change to allow each command to define a list of files or file patterns that definitively don't affect that test, so that CI can be smarter about what tests to run (e.g., not running expensive integration tests for README changes).

A side effect of this change is that tests of almost all commands now need a mock GitDir instance. This would add a lot of copy/pasted boilerplate to the test setup, and there is already too much of that, so instead this refactors common test setup:

• Creating a memory file system
• Populating it with a packages directory
• Creating a RecordingProcessRunner to mock out process calls
• Creating a mock GitDir that forwards to a RecordingProcessRunner

into a helper method (using records and destructuring to easily return multiple values). While some tests don't need all of these steps, those that don't can easily ignore parts of it, and it will make it much easier to update tests in the future if they need them, and it makes the setup much more consistent which makes it easier to reason about test setup in general.

Prep for flutter/flutter#136394

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
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• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[stuartmorgan-g]

You'll want to read the test/util.dart and test/mocks.dart changes before all the rest of the test changes since those are the foundation all the test changes are built on.

[stuartmorgan-g]

This code was only used in one place (and isn't really related to git so it's not clear why it was in this class), so I inlined it where it was used so that I could use the cached changed files list instead of recomputing it here.

[stuartmorgan-g]

This was the test for the now-inlined code. The existing command-level tests cover the functionality in that context.

[stuartmorgan-g]

I extracted a helper like this in a couple of tests. Some of our unit tests have the unfortunate pattern of setting up a usual command in setUp, but then in a few specific tests creating a custom command instance that overrides more things, ignoring the one in setUp, and using the local one.

I didn't want to try to fix all of those in this PR since this is already quite large, but when I hit those cases I did pull out a helper so that changes like "add gitDir: gitDir because now the command needs that mocked out" can at least be made in only one place in the file instead of having to look at why a subset of a command's tests are failing and then updating them with exactly the same change.

[stuartmorgan-g]

We no longer needed the parentDir version, so I was able to simplify this method quite a bit.

[tarrinneal]

Much cleaner than before. I like it.

[tarrinneal]

nice