fix: don't overwrite unexpanded git-lfs pointer files. (#607)

It's possible for those with incomplete `git-lfs` installations
(and many more situations) to end up in a spot where pointer files
aren't expanded. If we overwrite the with archives, files look
changed which can be confusing and lead to even bigger messes
to happen.

Now we don't overwrite those files anyomre.

tests/tools/src/lib.rs

@@ -417,6 +417,16 @@ fn write_failure_marker(failure_marker: &Path) {
     std::fs::write(failure_marker, []).ok();
 }
 
+fn is_lfs_pointer_file(path: &Path) -> bool {
+    const PREFIX: &[u8] = b"version https://git-lfs";
+    let mut buf = [0_u8; PREFIX.len()];
+    std::fs::OpenOptions::new()
+        .read(true)
+        .open(path)
+        .and_then(|mut f| f.read_exact(&mut buf))
+        .map_or(false, |_| buf.starts_with(PREFIX))
+}
+
 /// The `script_identity` will be baked into the soon to be created `archive` as it identitifies the script
 /// that created the contents of `source_dir`.
 fn create_archive_if_not_on_ci(source_dir: &Path, archive: &Path, script_identity: u32) -> std::io::Result<()> {
@@ -426,6 +436,13 @@ fn create_archive_if_not_on_ci(source_dir: &Path, archive: &Path, script_identit
     if is_excluded(archive) {
         return Ok(());
     }
+    if is_lfs_pointer_file(archive) {
+        eprintln!(
+            "Refusing to overwrite `git-lfs` pointer file at \"{}\" - git lfs might not be properly installed.",
+            archive.display()
+        );
+        return Ok(());
+    }
     std::fs::create_dir_all(archive.parent().expect("archive is a file"))?;
 
     let meta_dir = populate_meta_dir(source_dir, script_identity)?;