bootstrap: keep all cargo test files in dist rustc-src

Cargo tests use some files that we would otherwise exclude, especially
the `cargo init` tests that are meant to deal with pre-existing `.git`
and `.hg` repos and their ignore files. Keeping these in our dist
tarball doesn't take much space, and allows distro builds to run these
tests successfully.

src/bootstrap/src/core/build_steps/dist.rs

@@ -830,6 +830,12 @@ fn copy_src_dirs(
             return false;
         }
 
+        // Cargo tests use some files like `.gitignore` that we would otherwise exclude.
+        const CARGO_TESTS: &[&str] = &["tools/cargo/tests", "tools\\cargo\\tests"];
+        if CARGO_TESTS.iter().any(|path| spath.contains(path)) {
+            return true;
+        }
+
         let full_path = Path::new(dir).join(path);
         if exclude_dirs.iter().any(|excl| full_path == Path::new(excl)) {
             return false;