Add a function to calculate the SHA256 to the migration template

This is useful to only apply migrations when the contents of the file
are well known.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

.github/cookiecutter-migrate.template.py

@@ -20,6 +20,7 @@
 And remember to follow any manual instructions for each run.
 """  # noqa: E501
 
+import hashlib
 import os
 import subprocess
 import tempfile
@@ -98,6 +99,26 @@ def replace_file_contents_atomically(  # noqa; DOC501
         raise
 
 
+def calculate_file_sha256_skip_lines(filepath: Path, skip_lines: int) -> str | None:
+    """Calculate SHA256 of file contents excluding the first N lines.
+
+    Args:
+        filepath: Path to the file to hash
+        skip_lines: Number of lines to skip at the beginning
+
+    Returns:
+        The SHA256 hex digest, or None if the file doesn't exist
+    """
+    if not filepath.exists():
+        return None
+
+    # Read file and normalize line endings to LF
+    content = filepath.read_text(encoding="utf-8").replace("\r\n", "\n")
+    # Skip first N lines and ensure there's a trailing newline
+    remaining_content = "\n".join(content.splitlines()[skip_lines:]) + "\n"
+    return hashlib.sha256(remaining_content.encode()).hexdigest()
+
+
 def manual_step(message: str) -> None:
     """Print a manual step message in yellow."""
     print(f"\033[0;33m>>> {message}\033[0m")