Rollup merge of rust-lang#148451 - Enselic:tidy-fix, r=clubby789

tidy: Fix false positives with absolute repo paths in `pal.rs` `check()`

Fixes this bug:

#### Step-by-step
1. `git clone https://github.com/rust-lang/rust.git rust-improve-tests`
2. `cd rust-improve-tests`
3. `./x test tidy`

#### Expected
No tidy errors found

#### Actual
```
thread 'pal (library)' (837175) panicked at src/tools/tidy/src/pal.rs:100:5:
assertion failed: saw_target_arch
```
#### Explanation

Since the git checkout dir contains the word ["tests"](https://github.com/rust-lang/rust/blob/bf0ce4bc6816e3b9aaa52dc5fd47b8b5b2e0cd50/src/tools/tidy/src/pal.rs#L96), the `pal.rs` `check()` used to erroneously ignore all paths.

Author: Zalathar

src/tools/tidy/src/pal.rs

@@ -68,14 +68,20 @@ const EXCEPTION_PATHS: &[&str] = &[
     "library/std/src/io/error.rs", // Repr unpacked needed for UEFI
 ];
 
-pub fn check(path: &Path, tidy_ctx: TidyCtx) {
-    let mut check = tidy_ctx.start_check(CheckId::new("pal").path(path));
+pub fn check(library_path: &Path, tidy_ctx: TidyCtx) {
+    let mut check = tidy_ctx.start_check(CheckId::new("pal").path(library_path));
+
+    let root_path = library_path.parent().unwrap();
+    // Let's double-check that this is the root path by making sure it has `x.py`.
+    assert!(root_path.join("x.py").is_file());
 
     // Sanity check that the complex parsing here works.
     let mut saw_target_arch = false;
     let mut saw_cfg_bang = false;
-    walk(path, |path, _is_dir| filter_dirs(path), &mut |entry, contents| {
+    walk(library_path, |path, _is_dir| filter_dirs(path), &mut |entry, contents| {
         let file = entry.path();
+        // We don't want the absolute path to matter, so make it relative.
+        let file = file.strip_prefix(root_path).unwrap();
         let filestr = file.to_string_lossy().replace("\\", "/");
         if !filestr.ends_with(".rs") {
             return;