For up to 127 bytes length, InnoDB would use 1 byte for length, and
that byte would always be less than 128. If the maximum length is
longer than 255 bytes, InnoDB would use a variable-length encoding
for the length, using 1 byte for lengths up to 127 bytes, and
2 bytes for longer lengths.

Thus, 1-byte lengths are always compatible when the maximum size
changes from less than 128 bytes to anything longer.

Field_varstring::is_equal(): Return IS_EQUAL_PACK_LENGTH also when
converting from VARCHAR less than 128 bytes to any longer VARCHAR.