tidy: fire less "ignoring file length unneccessarily" warnings

This avoids a situation where a file is at the border of the limit,
and alternates between hitting the limit and not hitting it, causing
a back and forth of addition of the ignore-tidy-linelength directive.

As an example, consider the ignore-tidy-filelength of compiler/rustc_typeck/src/collect.rs.

It was added in 2ca4964, removed in
37354eb, added again in 448d076,
removed in 3171bd5, added in 438826f,
and rust-lang#94142 is going to remove it again.

To avoid this back and forth, we exempt files from the unneccessary
ignoring warning that have length of at least 70% of the limit.

src/tools/tidy/src/style.rs

@@ -395,6 +395,9 @@ pub fn check(path: &Path, bad: &mut bool) {
                 );
             };
             suppressible_tidy_err!(err, skip_file_length, "");
+        } else if lines > (LINES * 7) / 10 {
+            // Just set it to something that doesn't trigger the "unneccessarily ignored" warning.
+            skip_file_length = Directive::Ignore(true);
         }
 
         if let Directive::Ignore(false) = skip_cr {