Refactor change detection for rustdoc and download-rustc

[liwagu]

This pull request refactors the change detection logic in the build process by consolidating redundant code into a new helper method. The key changes include the removal of duplicate logic for checking changes in directories and the addition of a new method to handle this functionality.

Refactoring and code simplification:

• src/bootstrap/src/core/build_steps/tool.rs: Removed redundant change detection logic and replaced it with a call to the new check_for_changes method.
• src/bootstrap/src/core/config/config.rs: Added a new method check_for_changes to centralize the logic for detecting changes in specified directories since a given commit.
• src/bootstrap/src/core/config/config.rs: Updated the existing change detection code to use the new check_for_changes method.

Cleanup:

• src/bootstrap/src/core/build_steps/tool.rs: Removed the unused import git from the helpers module.

  r? @albertlarsan68

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @albertlarsan68 (or someone else) some time within the next two weeks.

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[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git pull --rebase https://github.com/rust-lang/rust.git master
$ git push --force-with-lease

The following commits are merge commits:

• 100ac12

[bors]

☔ The latest upstream changes (presumably #131063) made this pull request unmergeable. Please resolve the merge conflicts.