Bundler: update git pins declared on git/github blocks

[matchbookmac]

What are you trying to accomplish?

Fixes #16435.

GitPinReplacer only rewrote tag:/ref: on gem "x", git: ..., tag: ... declarations. Bundler also lets you declare gems inside a git/github block, with the pin on the block's call:

git "https://github.com/example-org/shared-gems.git", tag: "v1.1.0", glob: "gems/*/*.gemspec" do
  gem "gem-a"
  gem "gem-b"
end

For these gems the Gemfile was never rewritten, so the lockfile re-resolved to the same content and LockfileUpdater#updated_lockfile_content raised RuntimeError: Expected content to change!. In a grouped update, that one failure marks every group member as unknown_error, and no PR gets opened.

Anything you want to highlight for special attention from reviewers?

• Rewriter#on_block now rewrites the pin on a top-level git/github block call when the block body declares the targeted gem. It then calls super so nested gem declarations are still visited. Blocks that don't declare the gem, and non-git blocks such as group, are left alone.
• The existing kwargs-rewriting logic was pulled into update_pins so the gem and block paths share it. It also now guards against a call whose last argument isn't a node.
• GitPinReplacer is also used by UpdateChecker::FilePreparer#replace_git_pin, so block-declared gems get their pin replaced there too during resolution.
• Custom git_source(:name) helpers aren't handled. That seemed out of scope.

How will you know you've accomplished your goal?

• New GitPinReplacer unit specs cover: a single-gem git block, a multi-gem block next to an unrelated block (only the matching one changes), a github block, and a non-git block (untouched).
• A new end-to-end FileUpdater spec uses a git_source_block fixture (dependabot-fixtures/que in a git ..., tag: "v0.11.6" do block, bumped to v0.12.0). It asserts that both the Gemfile tag and the lockfile tag:/revision: change. Without the fix, this spec reproduces the bug: the lockfile is unchanged.
• Locally, spec/dependabot/bundler/file_updater* and spec/dependabot/bundler/update_checker pass (505 examples, 0 failures), and rubocop is clean on the changed files.

Checklist

• I have run the complete test suite to ensure all tests and linters pass. (ran the bundler file updater and update checker suites, not the full suite)
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

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