Rollup merge of #158552 - ferrocene:jyn/tidy-errors, r=clubby789,davidtwco

make some tidy errors around python easier to understand

someone i was helping hit a tidy python formatting error in CI, tried to run the exact `tidy` command that bootstrap printed as an error, got *another* error because rustc isn't installed globally, only locally, and then gave up in frustration because they couldn't tell what tidy even wanted them to fix.

make some targeted improvements.

i'm not sure why diffs were disabled by default before. it's been like that ever since python formatting was originally added in https://github.com/rust-lang/rust/pull/112482, with no explanation. i think showing the diff is a better default.

before:
```
$ ./x t tidy --extra-checks=py:fmt
checking python file formatting
Would reformat: ferrocene/ci/scripts/calculate_parameters.py
1 file would be reformatted, 118 files already formatted
rerun tidy with `--extra-checks=py:fmt --bless` to reformat Python code
tidy [extra_checks:python_fmt]: checks with external tool 'ruff' failed
tidy [extra_checks:python_fmt]: FAIL
tidy: The following check failed: extra_checks:python_fmt
Command `/home/ci/project/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy --root-path=/home/ci/project --cargo-path=/home/ci/project/build/x86_64-unknown-linux-gnu/stage0/bin/cargo --output-dir=/home/ci/project/build --concurrency=4 --npm-path=yarn --extra-checks=py:fmt` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1630:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1533:29

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:00:55
$ /home/ci/project/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy --root-path=/home/ci/project --cargo-path=/home/ci/project/build/x86_64-unknown-linux-gnu/stage0/bin/cargo --output-dir=/home/ci/project/build --concurrency=4 --npm-path=yarn --extra-checks=py:fmt --bless
tidy [CI history]: no base commit found. Some checks will be skipped.

thread 'deps (.)' (25771) panicked at src/tools/tidy/src/deps.rs:700:20:
cmd.exec() failed with `cargo metadata` exited with an error: error: could not execute process `rustc -vV` (never executed)

Caused by:
  No such file or directory (os error 2)

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

thread 'main' (25768) panicked at src/tools/tidy/src/main.rs:53:61:
called `Result::unwrap()` on an `Err` value: Any { .. }
```

after:

```
checking python file formatting
Would reformat: ferrocene/doc/specification/make.py
1 file would be reformatted, 118 files already formatted

python formatting does not match! Printing diff:
--- ferrocene/doc/specification/make.py
+++ ferrocene/doc/specification/make.py
@@ -64,6 +64,7 @@

     return dest / builder

+
 def build_linkchecker(root):
     repo = root / ".linkchecker"
     src = repo / "src" / "tools" / "linkchecker"

1 file would be reformatted, 118 files already formatted
rerun with `--bless` to reformat Python code: `./x.py test tidy --extra-checks=py:fmt --bless`
```
```
thread 'deps (.)' (249572429) panicked at src/tools/tidy/src/deps.rs:696:9:
tidy must be run under bootstrap (./x test tidy), not as a standalone command
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```