fix(client): merge concurrent appends at end of file

backend/internal/client/sync.go

@@ -254,13 +254,28 @@ func (c *Client) merge(ctx context.Context, local, base, remote protocol.File) (
 	cmd := exec.CommandContext(ctx, "git", append([]string{"merge-file", "-p"}, names...)...)
 	cmd.Dir = dir
 	merged, err := cmd.Output()
-	if err != nil || len(merged) > protocol.MaxBlobSize {
+	clean := err == nil
+	if !clean {
+		merged, clean = appended(versions[0], versions[1], versions[2])
+	}
+	if !clean || len(merged) > protocol.MaxBlobSize {
 		return protocol.File{}, false
 	}
 	hash, err := c.cacheBlob(merged)
 	return protocol.File{Hash: hash, Time: max(local.Time, remote.Time)}, err == nil
 }
 
+func appended(local, base, remote []byte) ([]byte, bool) {
+	if !bytes.HasPrefix(local, base) || !bytes.HasPrefix(remote, base) {
+		return nil, false
+	}
+	tail, glue := local[len(base):], []byte{}
+	if !bytes.HasSuffix(remote, []byte("\n")) && !bytes.HasPrefix(tail, []byte("\n")) {
+		glue = []byte("\n")
+	}
+	return slices.Concat(remote, glue, tail), true
+}
+
 func isText(data []byte) bool {
 	return len(data) <= protocol.MaxBlobSize && utf8.Valid(data) && bytes.IndexByte(data, 0) == -1
 }