of

The checksum of content, after normalising away the differences a checkout can introduce.

Normalised: a leading byte-order mark, \r\n and lone \r line endings, and whitespace at the very end of the file - an editor adding or removing the final newline is not an edit to the migration. Everything else counts, whitespace inside the file included: reformatting a migration that has already run is a change to a file the database has a record of, and saying so is the point.