merge: keep content before first header

This fixes the `-u` option for `apy add-from-file` and `apy
update-from-file` so they don't remove content before the first header
line.

refer: #119

Author: lervag

src/apyanki/anki.py

@@ -597,7 +597,8 @@ def _update_file_with_note_ids(
         note_positions.append(len(content))
 
         # Extract each note's section and check if it needs to be updated
-        updated_content: list[str] = []
+        # Keep content before first '# Note'
+        updated_content: list[str] = content[0 : note_positions[0]]
         for i in range(len(note_positions) - 1):
             start = note_positions[i]
             end = note_positions[i + 1]